How to find a Girl for my Boyfriend and I?

We're both 26yrsold..been dating for 2yrs. I want to bring a girl into the relationship...just to mess around with. I want to mess around with her...and I want her and I to tease my boyfriend. I think it would be fun! ;) It's my idea, and I don't want them to have sex, I just want to play around with her and then play around with my guy. Anyway, how do I find a girl willing to do this, how do I even bring it up? I've never really done this before...as in brought a girl into my relationship sexually. I'm not a lesbian, but I like to dabble. How do I go about this without seeming like a freak????????