Boost has been too much for the old cylinder head gasket ..
Is there different grades ?
I think I need one for high boost
Best choice /cheapest source ?

As long as it's the stock Elring, it's fine - don't get anything else. Some vendors do solid copper hwadgaskets, but given that every single report I heard about them was negative (won't seal properly after installation) I'd be wary about them.
If a new stock head gasket fails - even on a highly modified car - something else is wrong.

If you still feel you need more clamping force, I'd recommend having metal O-rings inserted around the cylinders in the block deck. That will hold at even the most extreme cylinder pressures (which you can't reach with the fuel available at your next gas, station anyway ).