Have you tried just searching the tire size? Searching bobcat a300 tires gives me a few options in the $1,000 range for 4 tires.
Are your tires filled or pneumatic? My mini skid steer has foam filled tires, all of them are completely bald and they still work great. In talking to other owners the bald tire helps with not tearing up the turf. You do lose a little traction, but if your not digging you shouldn't need it.