Aerobic Oxygen

Makes and sells an additive for purifying water.
http://www.oxygenforlife.net/

Arctic

Sells British Berkefeld and Doulton water filters and cartridges.
http://www.arctic-1.com/filters.htm

Black Mountain Stores

Sells Katadyn water purification filters. Also includes online versions of the original instructions.
http://www.katadyn.net/index.shtml

James Filter

Carries British Berkefeld and Doulton water filters and cartridges.
http://www.jamesfilter.com