If you are after cheap wood then it can be a good idea to go round local tree surgeons and see if they have any wood for sale, which should be reasonably cheap. The only draw back is that it will almost certainly be green timber and seasoning it yourself can take money or space.
Another option is to check round sawmills. Some sawmills stock both wet wood and seasoned wood but the seasoned wood is usually more expensive than the wet wood since you are paying for the fact it has cost the sawmill time or money to season it. If you opt for the wet wood though you will need to be prepared to season it yourself. If buying seasoned wood I find it usually works out slightly cheaper if you buy it in plank form rather than cut into bowl blanks.