The project involves making a papier-mache tree, about 50cm high. So, I need to make a wire armature first but I'm not sure what type of wire would be ok to use (naturally, I don't want it to get rusty inside or something). Perhaps copper?
I suggest aluminum : moderately corrosion resistant, inexpensive, easy to bend and cut, easily available. Look for it at farm supply store as electric fence wire. It does come in a fairly long spool but I find other uses for it like holding chicken and roasts on a rotisserie and in the garden holding branches etc. Aluminum is not very strong but you would have a lot of it so double and triple it for strength. And corrosion products are white so would not show unlike any copper corrosion near a surface which could cause blue/green stains. For limited size and number of items , insulated solid (not stranded) copper ( electric wire) should not be too expensive. Size near 12 gage should bend and cut without difficulty. Also coated steel wire is sold for bonsai plants but I expect it is expensive.