I assume you are wanting 408 CT or 375 CT cases? You can get them direct from Chey Tac Industries as well as from Jamison International who makes them for Jamison.
Run a search for either and their web sites should come up.
You can also get Bretram 408 CT brass from EDM which is sold for the rifles they make.
Allen Precision Shooting
Home of the Allen Magnum, Allen Xpress and Allen Tactical Wildcats and the Painkiller Muzzle brakes.
Farther, Faster and Flatter then ever before.
Web Page: www.apsrifles.com