tire size

I have 265/50-15 (sometimes) on the back of mine and it is tight. They actually wouldn't fit but I took the quad's off and reversed them. Can't even get a pencil in between the two. I'm looking for something like 245/55-15 or 255/55-15 when I replace them but can't seem to find those sizes.

EDIT: Hey, I'm a "senior member" now. lol.