#2499 fel-pro pro-ramic
Get it at lots of auto parts places like NAPA etc.
About 1.5 mm thick. Probably crush to about 1.2 or 1.3. Ceramic fiber coating both sides of a perforated metal sheet. The perf metal core makes it practically indestructible once it's in there and bolted down. Extreme heat tolerance. Makes really nice little exhaust gaskets too.
Not as easy to cut as non-reinforced gasket, but worth the effort. I use steel hollow punches with a heavy aluminum plate backing up the gasket. Works great. (edited)