I've purchased a bunch of pre-keyed stock footage of people here and it's not to expensive at all. I even have purchased a bunch of people dancing from here too. both front and back shots, men and women. Check it out because I think that's exactly what you are looking for.
There is also a video tutorial on how to use Trapcode Particular 2 or Trapcode Form to create a large crowd using that footage and doing what might be similar to what you are looking to do with the crowd. If you like, check it out to get some ideas: