when i purchaced a spare for my cardnal, i went to a trailer supply place. in my case, they hadn't seen a 6 lug 16" that was any spacing difference than what they had. i purchaced 2 w/tire and they fit.
if u have a 15", i would suggest pulling one of the wheels (tire and rim) for compairson. it doesn't have to be a travel trailer dealer. mine was bought from a utility and truck supply place.
the cost of tire and rim is less than if u buy a rim then go get a good tire. i would only trust their tire for a spare. after it gets a couple yrs old, i would replace with a better grade tire. I believe i paid $130 for tire and rim last year (memory is terrible and that was last yr).
on a side note, when u replace bearings or seals, take the old ones with u. some of them are too close to recall by memory.