I was really happy with my double thickness gaskets from GrimmSpeed. I don't have any for sale, but they do! It will be quicker than waiting to find some on here also. Minnetonka Subaru has 3 inch gaskets in stock also...I believe.
your best bet is to do what the guy above me said, find a local exhaust shop or performance shop that has them in stock. since i know me personally i only buy gaskets when i need them, and throw the old ones out.
Hmmm...you know, i don't remember for sure. Maybe $16? I wouldn't buy those unless you have a trouble spot though. Single thickness gaskets are plenty for regular exhaust installations.