Not sure if you are set on using cast or jacketed bullets. Hard cast can for the most part be driven faster and at lower pressures than jacketed bullets. Dave Jennings at Montana Bullet Works has a big variety of cast .458 bullets http://www.montanabulletworks.com/default.php. Also, try Beartooth bullets for cast .458 http://www.beartoothbullets.com/index.htm.