For 31s, either 15x7 or 15x8 would do just fine.

On my last Toyota, I bought an used set of 15x10, and the guy had 31x10.5-15 tires on them. I ran them for the next 2 years w/o any problem. Given the 10" wide rim, the whole side of the tires were very round, but never had problem with popping the beads or anything. It definitely gave a wide stance to the truck.

My 2 cents is to just buy some used 15x7 Toyota alloys as there are many on the markets for good price. But if you're buying new, I would consider 15x8 for the extra width in stability, and also if you ever want to upgrade (who wouldn't) in the future for 33" or 35" tires, the 15x8 would be perfect.