I don't believe any sealer is needed on the flat gaskets. I didn't use any
when replacing the muffler and there were no leaks.

If you need to replace a doughnut gasket (like in front of the j-pipe on
some configurations, I think), then you could use something like this:

http://www.international-auto.com/index.cfm/fa/p/pid/2767/sc/8140

- Kevin