Just like you wouldn't find parts for your Ford on the Ford website, you'd have to contact your local dealer. They MIGHT actually be less expensive than having one made - maybe
But with a local upholsterer, you'd need to take the boat to them - "possibly" they'd come to your boat and measure - depends on your area. Installing is simpler than making it so that could be done if the boat is still in the water.
Check both places.
Another source is whomever actually made the cover for Grady - maybe AJ Canvas? But your dealer or Grady white could tell you that.