less common tire size

I am looking for rear tire options for my mid 80's Snapper Lt12, it takes a 21x7x10 and i have found not much, It is a somewhat common front atv tire size and i am looking for ideas for a moderate price (under 100 a tire).

That is an odd size tire. I thought I would follow one of my links when I was looking for tires, but I came up with nothing. That led me to searching around a bit and pulling up some pictures of that Snapper. These are pretty close: ITP Mud Lite AT Mud Terrain ATV Tire 22x8-10 or Carlisle All Trail II ATV Tire - 22X9.50-10.

What is this tractor used for?

Yeah, like the above poster gave some different sizes, the common denominator was the rim size. As long as you know it's a 10 inch rim, you could do just about any size (larger or, usually less preferably, smaller) as long as it'll clear your fenders and whatnot. The first number is the second most important (next to the rim size ( third number)). Also, a 10" rim is kind of the standard for lawn tractors/mowers that a lot of folks here are building from. Perhaps checking some other threads will net you some ideas of what others have found and done. If you need that exact size you currently have, I can't help you. Chances are though, you could easily do something else/easier to find.

you can get any tire as long as its a 10 inch rim hole and you can get different rear rims as long as they have the same axle hole, I have a friend whos mower had 10 inch rear rims and swapped them down to a 8 inch rims so he can run 20x7x8 duro thrashers