I've always wanted to make electric guitars myself. However, it seems a bit hard to find the right supply of wood to do so, as musical instruments usually require naturally dried wood pieces for acoustic stability.
The properties of the wood, especially its density will have a significant effect on the tone of the instrument but there is quite a wide variety used as well as various laminates.
Of critical importance is the stability of the wood as even small amounts of warping can seriously affect play-ability, this is especially the case with the neck which needs to be able to resist the tension of the strings without bowing or twisting. Indeed the necks of most steel strung guitars are fitted with internal 'truss rods' which can be adjusted to balance the tension of the strings.
Similarly wood used for musical instruments needs to be very well seasoned this takes time and carefully controlled conditions.
One good option for a first project is that there are several companies selling guitar kits in various degrees of completion with a wide variety of body and necks styles and hardware available. If you want to design your own body shape you could either modify an existing style (the kits are usually clones of popular models/brands) of get one with a through neck so you at lest have the crucial mechanical parts done and the material for the rest of the body becomes less critical. Indeed there have been a few 'fantasy' guitars built around a through neck using foam and fibreglass to sculpt the body.
Another option is to look for salvaged wood. Brian May famously built his own guitar, with his dad, using wood from a fireplace mantle shelf and other sources. Hardwood tables and bar tops may also be good sources. These also have the advantage that you know that the wood is well seasoned. Chunky shelves and table tops are often made from mahogany which well suited to making a guitar.
Anyone with a large enough tree is a potential source -- but you might have to do some more work (square it up to make a blank & dry it to the appropriate humidity to work it)
When my neighbor had a maple dropped that was about 5' across, someone came up and asked if they could have some of the wood to make a cello. He had said that the trick was to paint the section of tree w/ latex paint to seal in the moisture 'til you're ready to use it. Unfortunately, he couldn't find a large enough section for his needs that didn't have some rot in it (which was why the tree was taken out)
You want to check with arborists or tree trimming services in your area -- they might be able to save you a suitable sized chunk from trees that they're removing, if the owner doesn't want it for firewood.