eBay is where I found mine, cost $300 used.
..some other possible sources:
http://www.viperforums.net/forums/forumdisplay.php?f=30
http://www.viperalley.com/
http://www.autopartswarehouse.com/search...on+%26+Steering,wpn_cat_name:Brake+Discs%5C%2C+Pads+%26+Calipers,wpn_scat_name:Brake+Calipers+%26+Components,part:Brake+Caliper)
http://www.donscharfautomotive.net/
good luck!