Are you wanting to replace the ones that are on it? The old style 3 bolt balloon tires are no longe made by anyone. Sometimes you can find one or two but they can be damaged easily and a lot are dry rotted.
Most people will buy or make a 3 bolt to 5 lug adapter and run a steel 8" beadlock wheel with 21" diameter tires. A good size tire would be 21x11x8. Carlisle Chevron or Goodyear Rawhide.
Richard Clark the owner of this site should have 3 to 5 adapters available and he also has wheel and tire packages that will fit the Terra Tiger.