The World Heritage Museum at the University of Illinois, Urbana-Champaign
had an excellent collection of casts when I was a graduate student
there.  Granted this was 12 years ago, but I would imagine they are still
somewhere on the campus.  We (the art history teaching assistants) used
them for teaching purposes.  I would contact the art history department
or the Krannert Art Museum on the Urbana-Champaign campus.  Good luck!