Hey Marc,
If you want originals, go to somewhere like NPD (national parts depot), classic industries, etc they can do you new seats. Or are you going for aftermarkets sporty style ones? like corbeau or recaro?
In regards to tan to black, grab some dye rattle cans.... wash and degrease the surfaces (mask off anything you dont wana turn black) and spary paint it. Technically you're not spray painting it as the spray has a dye so the black won't flake, it will turn it black. think i made my point
http://www.frost.co.uk/item_Detail.asp? ... ctName=VHT Penetrating Colour Dye Black (325ml)
MUCH cheaper than replacing and if you buy aftermarket and not OEM, fit may not be right and you may introduce squeaks and ill fitting areas, though may be wrong. This stuff can be used on leather, even carpets. Carpets you have to brush one way, spray, brush another way, spray etc.
Hope that helps!
Cheers,
Ray