I've bought tires at junkyards before in a pinch... you might want to call around. Though generally they stick you for the rim it's on too, but then you don't have to deal with the cost of swapping tires on your rims - only balancing.

Sometimes tire stores keep used tires that aren't too crummy looking for very little money, got a set of tires that way as well. Again, it's a matter of calling around.