Threesomes

Boyfriend and I were talking the other night, and the topic of threesomes came about.. he said he would like to try it once, just to experience something new. I kinda agree (always wanted to, never thought he would) but I don't really know how to find a third partner. I've heard people say to do it with a stranger then I've heard people say do it with someone you know. I would really like to surprise him with one for his birthday but how do I find someone else that wants to? Help, please!