The problem, and an important consideration. well, a bunch actually....
First, the shipping. Too large to ship UPS, figure on $100-$150 just for shipping. That's basically HALF the cost of an entire cover.
Second, the taper of the foam. Almost no manufacturer slices their own foam (we're one of the few) and orders the foam specifically tapered to their own specifications and based on what's available from the foam supplier they use (the variations from one manufacturer to another can be as much as 3/4" of an inch). If the taper isn't the same, and you probably wont be able to exactly specify it, the foam wont fit the vinyl jacket properly causing either wrinkles, or not being able to fit.
IF, you can get the foam from the SAME manufacturer that initially made the cover you have, and they're reasonably local to you, it could very well work.