Its always tough to find them for big sizes. Up to 35" I have taken them from manufacturers like BF Goodrich, etc, who publish the sizes.
Above that, the sizes really start getting somewhat different from the sidewalls anyway, so those are pretty close approximations.
If you want to calculate a tire's revs per mile its this:
Although, tire circumference isn't exactly the right term, since the tire bulges and has a lower effective rolling radius. So you calculate the circumference using the rolling radius.
If you have a 31" x 10.5" BFG AT on an 8" rim, it has a true rolling radius of 1.245 feet (14.94 inches). So the radius is 1.245 x 2 x 3.14159 = 7.82 feet.
Every rotation, therefore, will drive you forward 7.82 feet.
5280 / 7.82 = 675 rotations per mile.
So, if you went out and measured, say, a 49" IROCK, and it had a loaded radius of 23.5", then it would have a circumference of 12.30 feet. And would have approx 429 revolutions per mile.