Get the open base gasket.

Barring that, I'd cut the gasket with a razor blade/knife/Exacto to match the intake manifold.

Bolt the carb down, then remove it.
Cut the gasket to the witness marks left behind.

Seems you could find an open style base gasket at Autozone or Pep Boys or similar.
Normally I'd recommend a real parts house, but sometimes the gaskets are hanging on the rack near the carb section.

Having said that, a NAPA store is a good bet to have the part you need most times.