Are you trying to buy actual uniforms? How about putting together your own? All the same color/style slacks/skirts, same color/style blouses/shirts, same color/style scarves/accessories worn the same way by everyone, same color/style (except for male/female) shoes. If everybody is wearing the same outfit, the outfits becomes a uniform even if it was all bought separately.
you can find out if your local high school drama/theatre will loan uniforms/outfits to help you out. sometimes they will loan them. otherwise check out costume rentals. Even some private schools have a bunch of stuff. call...it doesn't hurt to ask.