it is certainly possible to build your own tubes but I'm not sure if it is the best thing to do. There is little chance of you being able to make them as good as the professionals and you can be sure it will be very time consuming.
I built my own 7m rib last year. I did all the build work along with a friend. The only two things we didn't make ourselves was the stainless steel work and the TUBES!
I would advise you to talk to Chris Hornidge at Henshaw Inflatables Ltd. They supply most of the UK manufacturers and the quality of their tubes is very good, also the price is quite reasonable.
The telephone number is 01963 33237 and the web address www.henshaw.co.uk
On the web page it tells you how to go about measuring the boat to ensure that the tubes are the correct shape.
Good luck with your project.