I had an ad on okcupid, plenty of fish, alt.com, adultfriendfinders you name it. I still get emails from them, which is annoying. There are poly dating sites too. But I don't have experience with them.
If you ask around people who date they will know what is popular for your area. Be sure to be very specific in your ad about wanting poly. There are more people into cheating, open relationships (ie. finding a one night stand), and swinging than poly. You'll have a lot of wading to do. Keep honest and patient. Take your time as there are a lot of one offs that think they know what they want, but when it comes down to it, just want to get laid and move on.
There are a lot of couples on those sites looking for the same thing as you. That doesn't mean it doesn't exsist, but can be very defeating.
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