Here are some Leviton part numbers.

L14-20 120/250 20 amps

Leviton number 2415 for a Flanged inlet

Leviton number 2413 for a matching cord cap.

But any manufacturer's NEMA inlets will fit any other manufacturer's NEMA cord cap.

I found a non brand name L14-20 flanged inlet for $21.00 by doing a Google search.

Bob Badger
Construction & Maintenance Electrician
Massachusetts