Actually, Alolan Vulpix is exclusive to SUN, not Moon. And it's also found kinda late-game at its earliest, before the sixth trial, in Tapu Village in tall grass. It also has a 10% encounter rate, so shouldn't take long to find.
You know, if you're interested, I can post a list of Sun/Moon version exclusives here, since you seem to have a dilemma with choosing, and it might be able to help you pick.