You can get 315's on a 10' rim, and that is pushing it. 315's won't work on a 9.5' rim. To do 315's right, you really need 10.5's.

 

I was told I can fit 315s on my 18x9s but the tire will bulge in the middle and not be as good as if it were on a 10 or 10.5 wheel by the tireshop i go to. He recommended me not doing due to it affecting my handling and the tire bulging. I found 315/36/18 on a couple websites for nitto DRs just do a google search for the tire size and brand you want thats what i did.

I have seen some cobra R replicas come in a 10.5, there is also soem BBK spider mesh wheels that come in 10.5, I know steeda sells them off there site and I think so did tirerack.com. Saleens come in 10s for the rear I think also.