I actually built my own Rib last year although it was out of GRP. It is definitely a good idea to let the professionals manufacture the tubes for you. I would recomend that you use Henshaw Inflatables who have a very good reputation.
I doubt if you are going to find plans on the net, but what I would suggest is that you find a Rib nearby that is roughly the dimensions you want and take a few basic measurements off it to get you started. You can then add your own modifications with bow height, chine positions, beam etc...
Good luck with your project!