Mike - I would have thought the major suppliers would have them. Typically called "High Back" I think. I know Moss lists them (fairly pricey), and Spitbits. I assume TRF and VickBrit would.
The highbacks are more money than the ones with detachable headrests, but then once you buy the headrest kits maybe it all works out.
I got a low-back upholstery kit from Canley Classics in the UK last year. Pretty reasonable, although shipping added quite a lot, and good quality. They list their sets for MkIV now at £137. I'm not sure if the highback seat was sold in the UK though, so these may be for detachable headrest seats.