Either:
1) An actual tractor dealer (like John Deere) will have them. Suprisingly, the Chicago area has a tractor dealer so they're not that hard to find. They replace or retread them like any repair shop and have to pay to have them disposed so if you're willing to pick them up they'll give you whatever they have.
2) A tire recycling place. I don't know if that's really what they do, but I drive by a place everyday where tires come and go. Every few weeks you can see one leaning against the wall for a bit.
Believe it or not, the big commercial gym that I go to once in a while (Lifetime Fitness) has one. One of the trainers got one to use with his clients. Since I don't have room for my own, when I want to play with it, he'll pull it out behind the gym and let me flip it as much as I want.
So they're not really that hard to come by...