What you are looking for is a NEMA L14-20 flanged inlet.
L for locking, 14 for 120/250 4 wire, 20 for ampacity.
Ask for this at any large supply house, they can order it.
No L14-20 is cheap but certainly you can beat $100.00
All manufactures use the NEMA designations.
A 15 amp duplex at a house would be a NEMA 5-15R.
5 for 120 volt 3 wire, 15 for ampacity and R for receptacle.