What's the best way to find a threesome?

I'm a single, intelligent young guy who is in search of a threesome with two girls. Of course, this isn't enough to find one and I'm wondering what is.

I currently have a girlfriend and I've expressed my want to have a threesome with her. She wants to, but is currently unconfident about it. However, she has given me permission to have one without her. I'm wondering what would be the best way of going about it?

Finding one girl on here is tough enough (especially when you're 22 and not 30+ - the main demographic here) but finding two would be far more difficult, I'm sure. I wouldn't even know where to begin.

What would you say is the best way of finding a threesome with two girls?