I obtained replacement heads from
http://www.magneticheadcompany.com/, talk to Joe Dundovic, a very good guy.
I bought heads for a 2 track Revox PR99. The heads don't look like the original Revox heads that are built "naked" and fit inside metal shields. However replacement heads for Technics, Teac and Pioneer decks are quite close to looks.
I have been more than happy with the Nortronics heads on my PR99. They actually outperform (slightly) a pair of very little used original Revox heads.