What are possible sources of wood pieces from for making musical instruments?
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.