This is my first time trying to make a costume out of anything other than cardboard and sheets. I wanted to make Greek armour out of real leather but my mum said that real leather is very expensive and very hard to work with. So I started searching for alternative materials. The problem is I can’t find a material with thickness and sturdiness similar to real leather. I’m only thirteen years old so I have a very limited budget. My dad says I should go with fiberglass but I’m very worried that it’ll be too stiff to bend over in. Any ideas?
What you're looking for is EVA Craft Foam sheets; these are the typical choice for costumers who want to create leather or metal-look pieces without using leather or metal. They're widely available from most craft stores, typically in the "kid craft" sections under brand names like "Foamies;" lately, more deluxe versions are also available in some stores in the newly-forming "cosplay" sections, often under the "YaYa Han" brand. The sheets are sufficiently thick to replicate leather, but soft and flexible enough to cut with regular household scissors (as opposed to using knives and leather shears). EVA foam won't be as durable as leather, but with care should hold up well, and there are a lot of tutorials available for crafting costume armor pieces using it.
You definitely don't want to do fiberglass; it's very difficult and hazardous to work with, and gets very heavy very quickly.
So for this you could use actual cloth (old sheets?), or perhaps felt, glued together with a non-toxic wood glue.
On the more "affordable" end, you could also try paper mâché mould of yourself. Or perhaps for the heavy-weight feel of bronze, make a breast-plate with plaster bandage. A combination of both might work well - a light substrate of plaster bandage, but then covered with a layer of cloth or paper mâché - to cover all the bandage-ness.
I guess it depends if you're after a "Looking Cool" costume, or a more "Rough and Tumble" version.
IMHO I don't think you want fibreglass, the resin and hardners are not nice to work with. And when it breaks, it makes for some nasty edges.
While working with leather can be difficult, it's a skill you can learn.
New, pristine leather is indeed costly, but go try going through local flea markets / second hand shops. It's likely you can find slightly worn out leather jackets or bags that you can reuse.
You might also find synthetic leather (which is made out of fabric coated with plastic). It's usually a bit softer and easier to work with.
A material made specifically to imitate real leather, with similar properties but for a much more affordable price is 'faux leather' (a.k.a. artificial or synthetic leather).
It is still pretty pricey, but probably the closest you can get to a real leather look without having to use actual leather. It's also a lot easier to clean.
Like the other suggestions here, it's a sturdy material, and might not be easily handled by normal sewing machines, but by hand it's quite easy to manipulate, and it comes in different thicknesses, so you should be able to find the right pliability/realism ratio.
Or you can even use a thinner kind to cover elements made out of cardboard or any of the materials mentioned in the other answers.
A disadvantage is that it's (usually) not porous, so it's not the healthiest material to cover your body with, but Greek armour is naturally quite breathable - which is logical considering the Greek climate - and usually consisted of separate elements.
Real leather is expensive, but it isn't particularly difficult to work with. Thinner cuts of leather can be sewn the same as most cloth and thicker leather just needs heavy-duty hand sewing (and a punch to make the holes). It has better dimensional stability (it doesn't bunch or stretch) than fabric, which can be good or bad depending on what you are trying to do. It is, however, very unforgiving. A sewing needle through cloth won't, generally speaking, damage or otherwise alter the cloth, but a sewing needle through leather will create a permanent hole; it needs to be in the right place the first time. You can patch a gash or compensate for an imperfect fit much more easily with cloth than with leather. Leather also has a non-uniform surface; brands, cuts, scars, and other marks. I consider myself fairly handy with this sort of stuff, and anything I make out of leather I first make with a less expensive material first.
EVA foam is a good approximation for the flexibility of leather for an equivalent thickness. If you have the opportunity, I recommend looking at real leather before trying to replicate the appearance with something else; it will give you an appreciation for how much it bends, the finish, the texture, etc. Modern leather clothing is one of the worst examples available; it tends to be a very thin skin of leather over a synthetic material. There is no substitute for knowing the material you're trying to approximate, but copious amounts of googling and youtube videos are better than nothing.
Even if you were to use real leather, you would still have a problem. When you scale down a design for a smaller body, you have to reduce the thickness of the material in addition to the length and height. Even real leather would require something to help with stability to create an "authentic" appearance.
If I were to try this, I would start with EVA foam. After cutting the pieces from 3-5mm (1/8in to 1/4in) foam and doing a test fit, I would glue a faux-leather fabric to the surface. The glue and the fabric should add enough rigidity to compensate for the thinner material. If this is having trouble holding the shape you want, I would use a fiberglass screen (any screen would do, but fiberglass is what I can get at the local hardware store). Cut the screen to shape, sew together the seams by hand as needed, and glue the screen to the back of the foam.
Another common material for costume armor is thermoplastics. I don't want to mention any brand names, but I feel this one can't be avoided. There are several brands, but Worbla is the most popular and often videos will mention it by name rather than by the more generic "thermoplastic". The thermoplastics I have used are thin, hold shape well, can take a texture, and paint easily. Thermoplastic over top of EVA foam is a popular technique as both materials need a little help to hold a rigid shape cleanly. It is, however, not particularly cheap and you need a heat-resistant workspace. I would probably turn to real leather before trying this, personally, but it is an option.
Metal or fibreglass lamellar.
This might be a bit above your current skill level, but you might consider making a suit of lamellar armor if you don't want to make a solid breastplate. This is a form of armor that consists of a large number of small plates made from a rigid material with holes punched or drilled in them, which are then laced together with some form of cloth string. This grants protection that is nearly as good as a solid metal plate, but with more flexibility and range of motion. The Wikipedia page has a pattern for one style of lamellar. A similar armor known as brigandine was popular in the medieval period, which consist of overlapping plates of metal that are riveted to the inside of a heavy piece of cloth.
Note that both of these types of armor would likely have been worn over some form of cloth padding for impact resistance, similar to how modern football gear has padding underneath the plates; this would likely take the form of the padded longcoat or jacket known as the gambeson or aketon. Depending on how authentic you want your piece to be, you might buy or make a genuine reproduction, or just substitute it for a modern-day padded parka coat or jacket.
For an example of what these armors might look like in real life, here is a Youtube video by a history-focused Youtuber discussing their new suit of brigandine armor, along with its historical use and its functionality.