I have never been with a woman. Lesbian bars are NOT a great idea. They tend to be full of paired women who don't like you looking at their partner. You could also take a look at the Advocate, which is a gay and lesbian paper, at the personals, and they also include advertisements for bisexuals. The nice thing about the online sites, though, is that they have pictures, and after all how someone looks is not an unimportant part of this whole deal, especially if you are looking for some sexual experience and not for a mate for life.
Some cities have bisexual organizations.
New York, San Francisco and LA certainly do. And probably other cities as well. Be ready though for a certain lack of enthusiasm from lesbians who see bisexuals as potential heartbreakers and don't like the idea of being used as fantasy material for one night of a woman's life. If you can find other women who are bisexual and not lesbians it'll probably be an easier experience because it will be a better emotional fit. Did you know that a study was done at the U. of Wisconsin on marriage versus live together relationships.