You can buy the gaskets from pretty much any of the big name parts houses. Auto zone, orilleys, and the like. Unfortunately, their websites seemed to have been written by monkeys and getting your hands on the right gasket proves to be quite a challenge. They are marked either not at all for location, just size, or are marked inconsistently. I have been trying to fix an exhaust leak I have for quite some time now. But have ordered the wrong gasket twice. I'm still not sure which one is the right one, and have almost given up.
But anyway, parts houses have them. Also, if you have a decent muffler shop you could get to, they may have some.(kind of a longshot--especially in Bum **** Iowa)
One more option is to just weld up the flanges, certainly not the best way to do it, but would work.
As far as a gasket after the collector, pre-cat, I'm not positive there is one. But, I would be inclined to think there was.
