An excellent resource for seasoned timber is old table tops. Sometimes they are given away free because they are scratched or the kids have drawn on them etc. The older the better, stay away from the modern ones as they are mostly veneered chipboard, compressed cardboard or something equally horrible with the thinnest wafer thin veneer.
I have had a couple of lovely bits of pine, mahogany, oak and ash this way. Yes you have to put a bit of work in to get the timber but if its free, its worth it! I have also been given some beautiful tables that I just could not bring myself to cut up so they were restored and resold splitting the profits with the original owner. Its a good way to learn finishing techniques too and if you are just starting out then the practice in squaring up the timber is invaluable, not to mention any financial gain :-)
Old solid doors can sometimes give rise to beautiful small boxes.