I bet you any old style cycle shop will have a pile of them under a bench. Mastercraft in Hereford still have the traditional workshop board with individual sprockets . Malliard turned into Sachs I believe?? and they used to make them. They don't break often, mine never have on a CX bike.
You're telling me you've never broken a rear axle with an eight speed screw on? I'm hardly the cannibal, but I've broken two on an eight, one on a six and even one on a five and that's just on road bikes. Never broken one yet on a cassette.