There are several people here who make custom belly bands. Check out the vendors list.
Oscar Newman's belly bands are fantastic. (full disclosure: I'm a retailer) Susan Lanci's are too. I only have one of her styles in stock, and a couple of sizes that might be the right fit. We are having a Labor Day weekend sale for 15% off everything storewide.
If you are interested, here is the Susan Lanci:
Susan Lanci Belly Band Bonenana Republic
The Oscar Newman:
Dog Bloomers and Manner Bands