You're looking for a 1 1/4" threaded Acme cap. I think this one might be threaded but you can always ask:
https://www.dkhardware.com/mec-me108...hoCvW8QAvD_BwE
A local propane supplier that also sells parts, may have something as well.
If all else fails, you could modify one of these to work as a cap:
https://www.amazon.com/GasSaf-QCC1-P...3865122&sr=8-8
You'd have to deactivate it by removing or modifying the brass fitting so it doesn't protrude into your tank's valve mechanism. Alternatively, I suppose you could just cap the 1/4" fitting.
Good luck. Let us know what you find out.