Hi, I've used Maxton Engineering for all my race suspension for years. They're really helpful and know the Mountain Circuit very well. Not only that, they always come over during practice week to help out competitors running their gear. Give them a call on 01928-740531 and they'll be able to quote you some pricing.
FYI, a remote reservoir shock of some description is probably what you'll need as the fluid takes a real hammering on such a long lap and the volume helps to dissipate the heat better than a standard format shock.
Good luck!