wider tires

Im going to be getting some wider rims soon. Gonna go up to 8 inch instead of the factory 7 inch on my sport. The new wheels will have a 4 inch backspace. I am pretty sure the factory rims are a 5 inch back space am I right? So this will put my tires out about an inch further out in the wheel wells. When I get the wider rims I also want to get wider tires. I am currently running the factory 235/75R15s. I have had 31x10.50s on it before and i liked them, but I was looking for something wider. I was thinking 31x12.50s but there are only a few companys that make them. Is there a standard size that is close to this that anyone knows of that may carry more brands?