Way back I used the Max Fisch guide or asked for recommendations in the MF forum but in recent years it's just been Google. Search terms like "professional dominatrix" "BDSM" and the city. Sometimes I'll add terms like "heavy bondage" or "extended sessions". For me, a good quality website with clear info, detailed equipment lists, services offered and images of both the domme (face not important) and the facility matters. A couple of short (less than 30 seconds) clips of the Domme either in session or speaking to the camera can also grab my attention and help in the decision making process