For potter fanfic, there’s plenty of online things for Hogsmeade, Diagon Alley & Hogwarts… outside of that, I do the same thing I do with originals. Select a real place, permute the name so it’s fictional, and that typically gives me close enough so I can add things as necessary. (Also, on the odd chance my story hits success with TV/films, keeps fans from flooding a real person’s house/business). Of course, originals and the potter basically take place in modern society, so that’s why my trick works. If it’s fictional-fictional, like game of thrones, well, all bets are off, but that’s not what I’m typically writing outside of dream-like sequences (or inside fictional video games).