I'm no expert, but here are a few suggestions.
OKCupid, like the above posters have mentioned, is pretyyt good, but like all dating sites, it's kinda meh...
Groups. Meetup.com, tribe.net, etc. There are a lot of group websites, some group websites specifically tied to your state or city (or county sometimes), so you may want to try a search for those and look for groups in your area.
REAL LIFE! If you're open about your polyamorous nature (you rebel, you!), you'll eventually meet people who like or share your way of life. This option is only for the mature and/or evolved, because it takes a lot of faith/courage to be yourself in the big world.
Events. This is realistically going to be your best untapped resource of like-minded people. Events where open-minded & liberal people collect are your best bet to find people who are in all different types of lifestyles. You don't have to feel inhibited or quiet at most of them because everyone is usually there for the same reason. These, and groups are going to be your major connection sources. Examples would be CES, anime conventions, Burning Man, raves, gaming conventions.
One very important thing to do is be honest about yourself. I'm in a... difficult relationship right now. But if I were like you and many other who are single/singleish and looking to meet people, I would BRING UP THE WORD POLY THE FIRST CHANCE I GOT. It whole-handedly makes them know you're into it, AND lets you know 1. they're not into it, 2. they're single and want to know about it, 3. they're into it!!! (YAY, let's go clogging together! lol)
Anyway, good luck and have fun!

Gabe