Yes, they are rare and owners often to want to hold onto them because they ride so well. Regretfully there are no modern equivalents being made. You will need to be patient, but I will let you know if I hear of any that are for sale. On the positive side, when fitted with a long seat-post, frame size is far less critical than with mainstream mountain bikes.
There are four catogaries of Geoff Apps bikes:
Pre 1985 Cleland Range-Rider's and Aventuras, (650B or 700C): These are lightweight but unrefined and very few surviving bikes have all their original components.
English Cycles, Jeremy Torr made variants, 1984-1988 (650B or 26"): These are easier to maintain though the brakes on the hub-brake variants are much more powerful than versions fitted with cantilever rim brakes.
The Highpath Engineering variants, 1985-1992, (650B or 700C): These are much more refined but can be hard to repair as so many of their components were custom made.
Cleland bikes made between 1985-2020 (Mostly 700C): These are uber rare and mostly owned by Geoff Apps' friends and family.