Mike -
There are custom cover manufacturers in almost any city of any size. Ask a boat dealer near you Or check something like here in St. Louis there is a yellow pages heading "Boat Covers and Upholstery"; Pat's Boat Covers is THE name in this area.
If that fails, look in the yellow pages under "Truck Equipment and Parts". (I'm referring to heavy-duty trucks like Kenworth, Peterbuilt, Freightliner......) Someone who makes tarps for the trailers on these rigs may be able to put together a cover for you.
me!