Polygamy

For some reason I feel like I want a polygamous relationship. Currently my gf is a stud and I'm a fem. I want another fem in our relationship. It's something I've always wanted to do, but wasn't ready for, but now I think I could do it. I just don't know how or where to go lol. Me and my gf have been dating only each other for the last 4 years so I think it's time! Anybody into this kind of thing? Or in a relationship? If so how did you find your 2&3 or 3rd. How should I go about starting this?