I imagine you want something bigger then what discount would sell you. I would try tire world or quality tire. Quality tire used to haul away most of our old heavy equipment tires when they were warn out. I'd guess you could offer to buy one of those from them.
The even cheaper route would be to walk in to a construction company's maintenance division and ask if they have an old tire hanging around they don't want. If you want to try that route I can give you the name and address to where I know you could get one pretty cheap. My old work. A guy came in one time looking for a tractor tire to put in his yard for his kids to play on. I think he gave my old boss $100 for it.