Who has successfully found a third and how?

My boyfriend and myself have been trying to find a third for a threesome or three way relationship. Only downside is that a lot of girls around our age are taken or not interested. Who here has had success in finding their third. How did you ask/find them.

We are 17 and 18. We had two people who were interested, but they both have boyfriends. Also the one lives in a different state so even if she was single it wouldn't be a possibility as of now.