Pipgirl I think it's easier to find couples to swing with than a girl out on her own. You would probably have no problem finding a couple who want you to join in but your boyfriend probably wouldn't go for that. Otherwise if you're looking for a free girl then swinging sites and the type are more likely to provide some results other than striking up a friendship with a willing girl friend. If your boyfriend is up for it maybe try finding a couple. I'm guessing he may not be up on watching another man with you? Some couples are up for the men watching the women. If you hook up with some one just make sure you set out the boundaries before you start. It's rude to change the rules on the fly. If you want the girls to interact, the boys to interact (macho feelings usually rule that out) or same room sex or whatever just spell it all out so no one gets angry or hurt.
Personally, I would go the very, very close, open mutual friend route. Then it's easy for all three of you to discuss it and figure out who wants what, and avoid any unbalanced expectations. I can't, for my own part, imagine having a threesome with my girlfriend and some newcomer. That seems like you'd be using them as a fancy sex-toy. I like partners, not toys. But, to each their own!
Its easier to find a guy. There are lots of guys out there who would like to fuck married women, even if the husband is there.