As you may have already guessed, woodworkers are always interested in trying to keep their costs down. Salvage, as the other posters have suggested, is always good and construction sites often have a lot of scraps they are glad to get rid of. Some woodworking prefers and almost demands the use of green woods and someone is always taking down a problem tree. It can either become firewood, bowls, spoons or windsor chairs. Estate sales often yield some good stashes, A lot of areas have folks operating sawmills where you can buy green lumber. You can save a bundle, and often get better quality and unique species by learning to air dry, and eventually kiln dry your own. Local new lumber is nearly always cheaper just because lumber is heavy and costly to ship. Always save the good stuff for projects and use the cheap or free stuff when practicing and trying out the tools. An internet search will yield lots of lumber dealers but that would not be my choice unless you just really have to have what they are offering. You can always use a small piece of rare wood as an accent instead of using it as the body of the project.
Finding the goodies is half the fun.