for those of you using copper gaskets. a great source is an tv repair shop. they use it for shielding electronics.
it comes in sheets about the size of a standard sheet of paper. very cheap.
thicknesses vary from thinner than a piece of paper to about a credit card thickness. if you know what thickness you will need, for squish or whatever), just bring a dial caliper with you to the shop, and match it up to a slightly thicker piece of copper sheet (to account for crushability during tightening).
a slight coating of spray copper on both sides seals the deal, like someone above has mentioned. note: a little bit of spray copper goes a long way and drips all over the place. the gasket can be reused many times, and is easily removable because the spray copper is not sticky and cleans easily.
-omb