I use
www.CDBaby.com, it's basically all underground/non-mainstream artists and they have all different genres.
You can search by "sounds like" too, just type in some well known artist you like, and they'll give you people who have a similar sound.
Since a lot of this stuff is underground, some of it's hard to find on other sites (if you're looking to grab it for free or listen to the whole album), put you can hear the first 2 minutes of most songs (which is plenty of time to tell if you like it or not), but of course they want you to buy the songs but it's all good because we need to SUPPORT INDEPENDENT ARTISTS!