Gosh, I see this questions everywhere, but no real answers!
Threesomes are definitely NOT for everyone. It depends on your current relationship and your personal jealously, security etc.
I'm bisexual and in my early 20s, and in a relationship wth a guy. His fanatsy, as he puts it, is to have "two of me", aka a threesome with another girl lol. Personally I've always had fanatsies of all girl orgies, so i think a threesome works for both of us!
I think you need to set up the boundaries before hand. I don't want to watch him penetrate another girl, I want it to be all touching and oral sex. He doesn't want to feel left out. We all have desires and boundaries that need to be respected.
As for finding that third person... I wish I knew how! I personally don't want a friend or anyone I know, first because there's no one i know that I am even attracted to, secondly, I want to keep this private. However, finding a stranger is difficult, what kind of girl wants to join a couple of strangers, literately? No one has answered any of my personal ads, and i have been posting them all over the internet for over a year now! I'm too shy to approach anyone in person, I feel like a creep!
So, we've come to option 3... we will hire an escort. We've got no other choice! (Unless I finally meet a hot bi chick!) The upside, is that its waaay more likely to be completely no strings attached, pure sex, and she will be "at our service", thus our boundaries will be kept. The downside, i'm not sure how STD-safe escorts are... and they are expensive!!!
But those are basically your three options... someone you know, a stranger, a prostitute.