Hi Simon,
In the UK there is a place called porschapart, they can be found online too.
For calipers, you could use anything that is 4-pot/aluminum. You can find these on the 944 Turbo and S2, front and rear, most Carrera 2 and 4s, though be aware that some Carrera 2s had 2-pot alloy rear calipers. The 928s had them too, though the front calipers are in the "Big Red" size and are black. Dimensionally most calipers are more or less the same yet piston sizes vary. The pads are interchangeable, though Porsche uses different compounds for different applications (front/rear 944/964 etc.). The 993 calipers are somewhat different but can also be used. Personally, I would not got through the trouble of converting to steel 944 or 924S brakes. If you don't mind spending some money on this, call a Porsche dealer and ask them for a price on 4 996 Turbo rear calipers, these are monobloc, have the pads resting on steel pedestals so they don't have the sticky pad problem the early calipers suffered from, and they have radial mounting holes which allows for a simple aluminum adaptor to fit them to a VW. These where the cheapest new Porsche calipers for a while. I have sold quite a few sets to the waterboys around 350 Euro a set, and the front calipers about E600/set, that was when the 996 Turbo was not out yet, but we had the parts documentation and I was playing with the pricing system. The calipers are more expensive now, but still won't be -that- expensive. They don't have to be painted/coated, they don't need any new seals/pistons or U-plates either.
The 996 Turbo rear-caliper is the nicest all-rounder for German Look cars IMHO, of course, there are a lot more options, Boxster-S, GT2 what not.. these are just nice
One set I sold is on a 66 Notchback on 993 (non-turbo) discs mounted on turned down VW disks (to make a hub) This car runs 16" Design 90's around them (very tight).
Hope it helps,
Brian