Welcome to the board
Most shops that make tubes should have a pattern for an Avon RIB.
Another option is Wing Inflatable's in Arcata up North who might be worth getting a quote from. Plus you could pickup and visit the area. I love hiking Tall Trees. I do believe they generally use polyurethane fabric for which they swear is superior to Hypalon. They do make tubes for commercial use including the military.
https://www.inflatablesolutions.com/
How old are the current tubes and why do they need replacement? Often wear patches can cover some damage. Replacement tubes are super expensive.