The Toys'R'Us of rims is www.racerwheel.com - literally thousands of rims. Nice website, fast viewing. You can view by what fits your car, check out their wheel & tire or 4 rim packages or browse them all sorted by manufacturer.
Something I have learned on here, and its just my opinion. But my research has led me to find that if you want the best performance go with 17" rims. A lot of people like the 17X9 up front and 17X10.5 out back. I like that combo myself.
Go with the 18" rims more for looks as from what I have read is that the 17 are better performance and 18"s sure do look good. Hope this helps
I have been looking at getting some 18"s mainly due to the fact that the body lines of the newer stang's have such a huge cut for the wheel wells, that those 17X8's actually look kinda small. Thats my opinion of course