There is a stock company that supplies generic footage of people with alpha channel masks already included, I want to say it’s VEER, but It may be someone else. Still, I too would probably “roll my own”, it would be faster, cheaper, and more closely resemble your actual vision of the shot because you arranged it.
In a pinch (like it’s late at night already and due in the morning)
you can use Poser to create a quick individual shadow or a whole crowd and render as shilouettes. This is good for one of those foreground elements of people in front of a stage. Old versions of Poser are free downloads and sometimes given away in magazines like DIGIT or COMPUTER ARTS. I think half my graphics tools at home came out of the free giveaway versions from those mags:-)
Too, you and one other person can be layered dozens of times in most compositing programs.