Even WALMART carries them - check out their RV section... Be sure you know which one you need... Your shore power cable will have the RV50A MALE plug on it so it will need to plug into a RV30A MALE PLUG to 50A FEMALE plug long adapter etc...
If you just grab one you may get home with the wrong ends to use...
One note is to always get the long DOGBONE type adapters. The small round plug type adapters is never a good idea. When they are used most of the time they will start getting warm to hot and then burn up your shore power cable plug end...
Roy Ken