Go to a place that does work on large farm and construction equipment. They will have some used ones and will sell you one or if you know a farmer he may have one sitting around in his barn or shed that he will let go. They are not cheap though as a new tire for a tractor can be $700-$2,000 so a used one that someone might want to keep or need for the purpose of having a spare will go for a couple of hundred.
Bunch of stuff here: http://www.christiansfitnessfactory.com
a tire shop?
mmawarehouse.com, mmaoverload.com, fighterwarehouse.com, or the brands sites like warriorwearclothing.com, or badboymma.com
i just realized you wanted tires, try a tire shop
Academy
online like mmagear.com
Go to a place that does work on large farm and construction equipment. They will have some used ones and will sell you one or if you know a farmer he may have one sitting around in his barn or shed that he will let go. They are not cheap though as a new tire for a tractor can be $700-$2,000 so a used one that someone might want to keep or need for the purpose of having a spare will go for a couple of hundred.