If you can find somebody who sells old Scopes, you might see if any of the old Bushnell, Tasco, Weaver caps fit.
If that doesn't work, you'll probably need to go see a Gun Smith. They might have some caps.
Have you got any new Scope's you own, that you could try? This will tell you right off the bat if a new Scope cap has the same thread pattern, or if you need a taller cap from at least same model of Scope.
Dearhunter: Today's Redfield is NOT the same Redfield from back in old days.