Many areas do have local shops that do various driveline, clutch and brake rebuilding and repairing. It would be worth an online or Yellow Pages search to see if there is such a shop in your area.
Barring that, new or NOS pressure plates are, as you've undoubtedly discovered, somewhat scarce and very expensive! (I'm assuming here that this is the original 6.25" coil spring-style clutch and not the later 6.5" diaphragm-style clutch.)
Rimmer Bros. in the UK shows them for $92.59 (not including exchange surcharge or shipping), but the picture they show is actually of a six-spring Herald 948-style pressure plate; the Spitfire should be a nine-spring plate (so I'd want to enquire to be sure of what you're getting). Canley Classics in the UK show them for $62.02 (reconditioned). Even with shipping, either is much cheaper than the $220-270 that the US suppliers are showing for these!
Sidebar: I made the mistake of reusing what had appeared to be a very serviceable pressure plate in my Herald (same clutch as early Spitfire). It may have been a fluke, but that pressure plate disintegrated, making a lot of noise, leaving a small hole and some other dents in the alloy bellhousing...and rendering both pressure plate and clutch disc useless. Fortunately at the time, I was able to source a new one on eBay at a good deal. They do come up occasionally, but not often and usually not cheap nowadays!
Update: a quick eBay scan shows:
3-piece kit; Buy it now! <$150.00
and
another "kit"; Buy it now! <$100.00 (enquire, as the illustration shows a diaphragm clutch!)