Finding correct gasket for aluminum valve covers.

Generally the steel and rubber overlay gaskets work best in this instance, and are reusable to boot. Edelbrock has some for $20 and Moroso's are pricier but highly rated. I think for a single use I'd spray those with some K-W or Fel-Pro copper coat gasket sealer, let it set up tacky, and run them until it was time to pull the covers off for routine maintenance.