An excellent source for used FGRV's: Molded Fiberglass Trailers | Fiberglass RV's For Sale
Might be a good idea to post more info about what you want, how much work (if any) you want to do, your general price range and how far you are willing to travel.
BTW: The newest Amerigo
FGRV is now 38 years old.
Also helpful is mentioning what you will be towing with.