For high temp and high strength gaskets the best that I have found is Fel-Pro Pro-Ramic 2499. Its a high temp gasket material that has a perforated metal core. Bought a sheet from NAPA. Not easy to cut into delicate shapes like head gaskets, but can be done with care.
“Fel Pro’s High Temperature Facing Material is Mechanically Clinched to Both Sides of a Perforated Steel Core. This Material is Recommended for Use Exhaust Manifold Gasket & Exhaust Pipe Flange Gasket Applications.”
We have a local tractor / lawn mower parts store that carries every gasket material you could need and you can buy any size you want by the square inch. It’s a really handy place to have.