Where can I find a blockoff plate for my 1996 era external fuel pump/sea water pump? I'm going to an electric EFI pump and only need the sea pump to be there.

The block off plate for an engine block will probably work.
You may want to consider using the mechanical pump to fead the electric pump though. That is what was done on a lot of the EFI setups of that era. Some electric pumps have a tough time pulling the fuel from the tank.
Another option is to sell that sea pump and buy a unit without the fuel pump. You may even make money, those things are big $$.

I bought mine at the local auto parts store.

Just to clarify- I'm going to a whipple EFI setup. The SX pump (similar to areomarine 1000) has an-12 fittings on it. Won't the mechanical fuel pump starve that huge electric model?

Maybe.
I think the mechanical pump will fead enough for about 600HP.
Do what ever Whipple tells you to do.