We didn't do a threesome exactly, but me and a girlfriend of mine got down and dirty a couple of times in front of both our husbands. It was at their request and they just watched and learned.
But I also am looking for an unattached woman who would be interested in playing with us. I have thought swinging might be better just because then everyone is already attached and you won't have to worry that someone would get too attached to you. When you find a single girl, there could be that possibility.
If you only want to do it once, I would say a professional would be best. Althought how you really find one, I have no idea! If you think it would be a long term thing, then you should try one of the friend finder type sites and take your time. I'm still deciding which of these paths to take myself.
I've thought about flirting with girls myself, but I'm not sure how to go about that. I've never done it, and it's kind of scary to think they might flip out on you or something. I guess I'd feel really butch doing that too, and I'm not like that.