Any aftermarket SR20 headgasket will work here as long as it has 3 bolt holes across the front of the gasket, like the GReddy one pictured below.

You can also use the OEM SR20ve or SR20vet metal head gasket found here.

We also stock Cosworth head gaskets found here.

If you choose to use a metal headgasket you must make sure that both the head and the block have been machined flat other wise it wont seal.

As for the thickness, please talk to your engine builder as this will depend on how much material was removed when the block and head were decked and what compression ratio is required.