I used PVC tarp for the inner and got it from www.profabrics.co.uk. They do heavy weight tarp and its cost about £8 per metre.
www.kayospruce.com do PVC in various weights, but seem to be more expensive.
The heavy weight tarp I used made the bag quite stiff and it was difficult to move around the machine when I came to sew the edging tape around the flap/opening. So I may get some lighter weight tarp for my next bag.
I'm also on the hunt for a presser foot that will make it easier to sew on the grosgrain edging. I can't seem to find one for my husqvarna machine. I did a near perfect job with the regular foot... but it involved too much concentration.
From here.