There are two styles of gaskets that COULD be used. The typical one is a flat gasket. These work great when there is enough flat surface to seal to. As weazler pointed out these joints are usually badly rusted. If the metal on the flanges is good enough to bolt to you can get a "donut" style or metal o-ring to seal up the joint. Get new bolts. Dont even think about getting the old ones off, just saw them in half.
If your cat has never been replaced, consider doing it. I got one for $90, so they arent ridiculously expensive. Unless you can weld, avoid the "universal" ones.
-SP