You can buy the bags but I've always just made them from plastic and packing tape. I'm sure there are much better ways but this has always worked for me. Vacuum bag material and the black stuff that you use to stick the vacuum bag material down would work too. Remamber to make the bag a little larger than the size it has to inflate to to make sure the mould takes all the load and the bag is just a seal.
I've done this on a smaller scale (1" OD) but used Latex tubing as the inflation medium. It's inexpensive, inflates easily, stretches a TON, the pneumatic fittings are very cheap, and epoxy doesn't stick (very well) to the latex tubing.
