wood sources depend quite a bit on where you live. I like to shop several local hardwoods suppliers, but there's also a couple 'plywood' stores that cater to cabinet makers, and these have a limited selection of excellent woods
if you're looking for certain specialty woods (like light weight Ash, which is marketed to the instrument industry as Swamp Ash and selected solely by weight from amongst several species) you may need to utilize an on-line source
http://www.woodfinder.com is a great tool for locating suppliers near you that have taken the time to post their inventory. be cautious, though, as some keep their inventory updated while others simply have a generic listing of woods that they carry on a regular basis or can order
all the best,
R