Sites for finding local poly groups in your area:
"As for where to meet poly people, if by some chance you are interested in anything alternative like Renaissance fairs, goth culture, sci-fi conventions, indie music, bdsm, or any small fringe group, you will be more likely to meet people who have at least heard of poly and are accepting of it."
-- SpaceHippieGeek, Polyamorous Percolations
Even if it's not an "alternative" type group, if there's a club or something in your area that does something you're interested in, you can always join that group and it just gives you a way to get out there and meet people. If you meet someone on a platonic level and get to talking about poly, then they can decide how they feel about it without any "pressure to agree." Then if they do decide poly doesn't bother them too much, and some kind of romantic connection subsequently develops, you'll already have "had the poly conversation" with them.
The dating sites, though (listed above), would be the places to place your ad.