How find third person for threesome???

Ok so I know this subject does fire up some people but my SO have been thinking of having a threesome 😱😏. Yes we have been very open and communicative on the subject and set boundaries and what not, the only problem we have is finding a women to do it with! I've tried dating apps that are specifically for threesomes and did a little pondering on Craigslist ( yes we will all get tested beforehand to make sure we're all clean ) but we've had no luck 😒! And we didn't want to go in Tinder or POF seeing that we know so many people on there and don't want our business out there. So I'm hoping you ladies who have had experienced this would be willing to give their recommendations on how to find our "unicorn" lol. Any advice would be appreciated 😊