If you want an original , you would have to order at a Bennington dealer. They do not have the snaps installed. Your best bet is for a local canvas shop to fabricate one .
You can also order a factor cover directly from Tumacs. Its not cheap. But if you don't have access to a good local canvas shop, then this is another route. As Jack said, I would explore local shops that can make one.
I went the Tumacs route only because we had no local shops around and I didn't want to trailer it 2+ hours away and let it set outside while a shop got around to making the cover.