So the best songbook I've found has been
The Fat Lute . It has a lot of great songs to chose from.
If you click on "Song Book" tab it'll take you to the pages where all of the songs are listed. You can sort the songs by different qualifications to find what sort of song you are looking for. I'm not too impressed by the search option because the way people title things is not standerdized.
The most useful sorting of songs is to sort them by parts. (Clicking on "Pts" should do the trick.) Once you do that you can find things for group music a lot easier.