Source for tires to see what fits?

[dontlifttoshift]

On a more serious note, you have to know approximately what you are looking for. Craigslist, the local autocrossers, and tire shops are all loaded with used tires. I have stacks of used tires here, you are more than welcome to take as many as you want.

If it's a toss up between 275 or 285, put the 275 on there and see where you end up, it's just a bit of math once you know what room you have.