I've done something similar but it wasn't with a boyfriend, just a friends with benefits kind of relationship. The fact he is your boyfriend will obviously add a different dynamic so it is great you have thought about that but continue to keep thinking and talking about it.
When I did it we had a kind of hierarchy. The guy was Dom, the other girl was switch and I'm sub so basically the guy was at the top of the hierarchy and he kind of instructed the other girl to some degree although she did have autonomy to do her own thing too. I didn't know the girl beforehand but I emailed her for a few weeks before and got to know her a bit before we met. The guy and girl already knew each other and for us that was quite important. I'm all about the dynamics between people.
We all met up at his house and spent quite a while talking. First just about general stuff to get comfortable and then about the details of what we wanted and didn't want. We'd already discussed this before but it was important to do it again. If any of us felt it wasn't right for us we could have backed out at any time with no hard feelings. This is really important.
I'm actually straight and had no experience with women so there were things I really didn't want to do like going down on her. A big part of it for me was doing things I didn't really want to in order to please the guy and to feel controlled but I had my limits and I made them clear. I get off on the power dynamic so although I have no interest in women, I definitely wanted to do it and enjoyed it.
There are a few questions I would ask myself if I was going to have a threesome again and perhaps they might be useful for you:
-Do you want the other person to be someone you know or someone you won't have to see again? Why?
-What would be the ideal dynamic? Do you want the other two to be a team or should one have more control of the situation (which is probably the wrong word, you should all be in control of what happens)?
-Do you want to see your boyfriend have sex with the other person? Do you not want him to have sex with them at all? Do you want to be forced to watch or just hear it? Why?
-What things would you want to include or not include? What words or actions do you dislike? Remember that your boyfriend knows you well, he will know not to call you x or do y but the other person won't.
When you know exactly what about a threesome interests you, you'll be more able to find someone suitable. I'm a big fan of degradation so I'd need someone willing to do things like that and who understands that I am straight. Everyone has their own kinks so make sure you know exactly what about the situation turns you on and find someone compatible. It is much better to take your time and get it right rather than going with the first person who comes along.
As others have suggested, kink events might be the best place to find someone but also just to learn more about why it interests you and learn lessons from others who have done it before. Other options for finding someone might include people you already know (if you have kinky friends) but that can sometimes cause issues and the Internet (there's fetlife which isn't a dating site but I think most of the half decent kink dating sites have disappeared).
The main advice I would give is think and talk about it lots and don't rush. I'm guessing you've had this fantasy for a while so waiting a while longer isn't a big deal. It's more important to do it right rather than do it right now.