I found mine in some woods that are in between a golf course and the road. It had been laying there for probably a year and one day when I was on a walk, I decided that I wanted it. So I rolled it all the way back to my house (about 0.5 mile) I'm sure the cars that drove by had no idea what to think, haha. Also, I was shirtless to add to the "crazy dude" factor, haha.
Maybe check the dump. If the attendant is in a good mood, he might let you salvage a tire. You could also check a farm supply store, although it'd be a shame to have to pay for a tire you're just going to beat the crap out of. Or freecycle.