The bolt pattern is 6 bolts on a 5.5" circle, 15" rim. Are you looking for a matching rim, or just one to keep as a spare that you don't mind looking different if you have to put it on the trailer? If you just want one the right size and bolt pattern, you can get one with a tire already installed from places like Northern Tool or a trailer supply. If you want one that matches your current rims, you'll probably need to find a used matching one.
Oh, and welcome to the forums!
Il Carriaggio — 1975 Argosy 24 | Il Progetto — 1976 Argosy 28 Center Bath | WBCCI# 15566
He has all of the virtues I dislike and none of the vices I admire. — Sir Winston Churchill