I recommend talking through the issue with your boyfriend and trying to cover all the bases of things that might come up during the threesome that could cause problems. Then talk about what type of person you both want so you're on the samne page when you take steps toward approaching people.
After that, I'd recommend getting a profile on OkCupid if Craigslist isn't your thing, and you don't have people in your life already who might be game for it. Check out other profiles of people looking for an additional person for their play time, or for people who are interested in being an additional person, just to see what's out there and how they describe what they want. Use those profiles to help you polish your own.