Paul since you asked I'll point you to junkyards, mechanics, even amusement parks that have 'tubing' attractions. You will need an inner tube from a larger tire, truck, bus or something.
My experience of rubber bags are from zampogna, they were popular for zamps) before the advent of goretex -like material). Rubber;s VERY bad for mouthblown pipes. no where for the condensation to go, so 100% effective at rotting wood.
Rubber pipebags are made airtight with repeated applications of automotive sealing caulk on the inside, and duct tape on the outside. Staples, stitches, organic glue, dont work. The only good thing about them is the ease of tying in stocks...just cut a hole smaller than the stock and the rubber stretches and seals up rather well around the stock. Tie a strip of innertube around it & your good to go.
Playing on rubber bags reminded me of riding a bus...always a bumpy ride..controlling the bag pressure's not intuitive either. Can hardly imagine what a pain this must be for uilleann & other dry blown pipes....Im glad to have rid of it, but if you want to try one out, my advice is to get in good
with your local michelin man; or go to the auto "u-pik-ur-part" graveyard