Summit also sales earls gaskets for headers and collectors, i don't know about the collector gaskets but the hearder gaskets are good. they have an aluminum frame and a type of gasket material in the centers for a seal, which are replaceable if they ever blow, the collector seem to be the same style.
Don't get the copper ones. I had some on my long tubes and had nothing but leaks with them. I since replaced them with some 'softer' ones that the an exhaust shop had. So far no leaks. Really surprised me, I thought the copper would be good. :?
A friend of mine gave me some collector "Donuts". I don't know what they are called. They look like Stock exhaust manafold donuts sort of but they are only beveled on one side and a straight piece of pipe on the other. He called them Header Donuts. They fit inside the headers with the straight piece and you connect the pipes to the beveled edge. Never ever change them again and they've never leaked.
I used several of the other type gaskets on the collectors and they all burnt or went bad in no time. These just work. He has a garage and had them in a coffee can with lots of other types of exhaust donuts. Some looked like they'd work just as well.