That was my other alternative for the potty. Also wanted some to make a bag for the folding loo seat so that I could have a 'wet side' pocket for soggy clothes and a 'dry side' pocket for clean clothes!
Its very reasonably priced at Pennine outdoor, they supply waterproof breathable material for people who want to make their own outdoor gear. Sorry am feeling lazy, if you google it their website will come up. THey do something called 'napwrap' meant for nappy covers which I recall was almost CHEAP!