Cladco in Rutherglen can do them, at the time I enquired they would only sell in batches of 10 curves or more due to setting up the machines (or something like that) this may have changed recently though so give them a shout and see. Failing that, I have two curves sitting here - they've been used and are a little bashed but should straighten up quite well. They're green coated and have an anti-condensation lining on the inside. Cost me £45 each - I'm in Lesmahagow (10 minutes from M74) if you're interested please tell me your address and I can send you photos/more info.
HTH
Karen