How do my girlfriend and I find another girl for a threesome? Is there an easy answer?
The classic method is to go to a party where there's a lot of heavy drinking, find the wildest girl there, and start dancing with her. After you get her into a sandwich between you and your girlfriend, start nibbling her ear and whisper some salacious suggestions. If she says yes, make sure you can get her to a horizontal surface within 20 minutes or less. Have drunk crazy sex, then pass out. The next day, hand around some aspirin and coffee with a little brandy in it for hangover remedy purposes. Everyone expresses some good-natured confusion about exactly what went on so that if anybody needs to pretend they don't remember, they can. Be sure she gets home safely.
This method sometimes results in gossip, hurt feelings, stalking, guilt, breakups, impotence, or vomiting rather than hot sex. You take your chances any time you do this, of course. You want to do your best to negotiate clearly and pick somebody who is happy and willing to engage in a threesome, but there's no guarantee things will end with sunshine and thank-you notes. But let's remember that even in a one-to-one encounter, the same risks exist.
Another possibility is to make a list of your friends, decide which ones you find attractive, and just proposition them—without the preliminary, inhibition-reducing effects of beer or cocktails.
Putting sex ads up on cragislist or some other venue is another alternative. Although single women who would like to engage in a triad get a lot more replies than couples who are seeking a third party, you may very well get lucky and find someone cute and compatible. Just be careful giving out too much information to a stranger. Meet your ad respondee in a public place before taking them home, and be sure to protect your credit card info and anything else that might assist in identity theft or robbery of your home.
Some people choose to hire a sex worker. You may have a negative response to this initially, and I think I can predict what your objections will be. You're afraid a sex worker will not genuinely enjoy herself, she'll just be there for the money, she might have a sexually transmitted disease, and she's probably got a drug problem. While any of these things might be true, they are also based on stereotypes. Getting paid for sex doesn't prevent enjoying it. A sex worker specializes in pleasure, knows what she is doing, and wants a date with you. If she isn't happy with the idea of bisexuality or group sex, she'll turn you down. And a sex worker is much less likely to present you with messy emotional after-effects than a friend. Many sex workers are actually happy and excited to have a female client, and will give you extra attention.
You should take precautions against sexually-transmitted infections with all your sex partners—amateur or professional. See my comments above for protecting yourself when you meet somebody new, whether that person is answering a sex ad or calling you back from an escort service. Remember that a hotel room is always an option rather than entertaining at home. (Hotel rooms always make sex seem extra-dirty, illicit, and good in a naughty way.)
Good luck in your quest for sexual adventure and wisdom. I hope you and your girlfriend find someone provocative and imaginative, and all of you have an evening you'll remember for the rest of your lives!