Watch Gecko, Straps Co - the usual suspects.
Most people would be pretty lousy at designing Nato straps, so the service you are after does not exist.
Furthermore, its not going to be as cheap as you think it is. Nato straps are woven in 500 - 5,000m lengths before they are heat cut and assembled. In widths from 18-22mm. Its not something you can make as a ‘one off’.