okcupid.com is a dating social network website. It is moderately accepting of polyamory. If you are married, you specify you are looking for friends. However, you can put search terms in your description that may be used to do searches like "polyamory." It is a free service as well. You could also try craigslist.
It is hard to say what cities may have polyamory meeting groups. Sometimes you have to go for a frienge group that is pro-polyamory like sci-fi convention, comic-con, or some kink based group.