what kind of crowd noises? just people talking, cheering, booing/hissing?
if it's just people talking, or whatever, you can do that yourself. Record like 10+ people saying anything.... random conversations, whatever. then layer them all on each other. Split each one in the middle and put the end in the beginning/beginning at the end. So now you've got 20 people take each and copy, and move the copies over a few seconds.
That should be crowded enough... if not, record more people, and/or keep splitting and rearranging to add more "people"