As far as a reliable provider of stone, the guys on ebay are a safe bet. You might also want to try looking at the rocks at your local "landscape materials" yard. I have purchased lots of knappable rock at these places, for cheap.
The best stone, however, can only be gotten by becoming part of a flintknapping group and asking those guys. If you don't want to spend any money, they will usually let you pick though the flakes on the ground... and take all you want. You can also trade for stone on this forum and others.