For the enclosure, I'd contact A&J canvas in Vanceboro NC as they make them for GW. If you have the existing set, mail them down for patterns.
Foe the cushions, any marine canvas shop can make em up but you won't get the GW embossing. I believe Llebroc is making all the cushions for GW now. You may be able to get them through a dealer with a mark up on them but Llebroc will not sell directly to the public.