Congratulations, though really there are no good parts sources that I know of... maybe in Japan though I wouldn't count upon it.
The fact it's the later watercooled hardtop doesn't really help since they adopted the Life drivetrain which wasn't widely exported either, and about the only body panels they shared with the earlier air-cooled Z360 & 600 was bonnet, roof & rear hatch.
Curiously I noticed that our front bumpers are also different from the Japanese spec, actually having their rear edges trimmed down to fit from the earlier air-cooled ones.
Often it comes down to experimenting. Some of its parts were shared with the early Civics (like drum brake internals) and I found an EG Civic thermo-fan switch fitted even better than the original (they had a reputation for overheating & popping head gaskets when these failed, not helped by tiny cooling capacity & the sensor being on the cylinder head instead of radiator like Civic). Bursons had even had a listing for a timing belt... if they can still supply.
Honda might be worth trying though some have suggested their motorcycle or even power equipment (lawnmower/generator) distribution networks can be more considerate if you can supply part numbers to check out (rumors there was also a generator that shared some engine parts).
Thanks for the heads up re parts. Most of the mechanical was sorted before the Z got parked up (been in hibernation for the last 20years). The only problem recently discovered was a rear brake had bound up. I'll do an update on here once I get the Z home.