I'm no expert, but here are a few suggestions.
Okcupid like the above posters have mentioned. It's prety good, but like all dating sites, it's kinda meh...
Groups. Meetup.com, tribe.net, etc. There are alot 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 alot 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 like-minded, open & liberal people collect, are your best bet to find people who are in all different types of lifestyles, and you don't have to feel inhibited or quiet at most of them because everyone is usually there for the same reason. This 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 were like you and many other who are single/single-ish and are looking to meet people, I would BRING UP THE WORD POLY THE FIRST CHANCE I GOT. It whole-handedly makes them you you're into it, AND let's 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