Also, don't bother with getting a gasket: make your own! Copper gaskets seem to work the best and I found how to make them very easily.
They sell copper sheets at the hobby store, about $10 for 6"x12" sheet. Trace the flange to make the gasket shape, then anneal the copper using a blow torch and water. You heat the copper until it goes dull red, then dunk it in the water. It softens the copper like you wouldn't believe, in fact, the gasket will easily distort and bend so you have to be careful.
Then, install. When you tighten the bolts, the soft copper will squish into all the irregularities in the surfaces to make a perfect seal. The first time it gets really hot, the slow cooling of the copper will harden it back to "normal" copper. If you need to refit the gasket, simply anneal it again!