I've been to private CGs twice that had 50 amp only posts. They both offered
to loan me an adapter if I needed it.
I have my own and by golly I paid for it and I was gonna use it!
FYI- WallyWorld used to carry them in the RV isle but my local store no
longer has them and they show "out of stock" online! This is for the pig tail adapter. You can get the 50 to 30 straight adapter at some WallyWord stores.
-2nd FYI- you are looking for the 50 amp MALE to 30 amp FEMALE adapter.
They can be had both ways so pay attention to the one you buy!