Before you find someone to join you guys I would be talking very in depth about what you hope to achieve and the rules of the engagement. Things can get pretty nasty when you bring another person into the relationship, even for just one night. You need to have rules and stick to them. You need to be very open and honest with each other and don't tell the other person what you think will make them happy. You will essentially be watching your partner be intimate with another, even for the pure thrill of girl on girl action it can be very difficult to deal with.

As for bringing it up? You can usually get an idea of what friends are either curious of dabbled in this kind of thing after a few drinks at a party for something similar. I don't know if I would suggest a friend or a stranger - both have their pros and cons.