Iain
First i would try your normal vinyl supplier, they would be the best to do you a deal.
most of the large suppliers will supply diamond grade, and will be around the same price range. You are better giving them a call and trying to negotiate a deal, but few will give much discount unless you already hold an account (my experience)
It is also possible to get diamond grade in ready cut kits and edge sealed, which is a route to go if you dont have the machinery for cutting it yourself
cant think of the people that do the kits but a google usually spits them out.
Peter.