Check yellow pages or look online for used tire places in your area. You can also check junkyards in your area.
Just inspect the tires they want to give you, I've bought used tires in the past that were damn near knew and had them try to give me ones before that had, at best, a few thousand miles left on them. You also want to make sure they aren't selling tires as used which have been plugged. Recapped tires are OK, plugged tires I would pass over.