'Topps Tiles' near me, in Reading, have an area out back for their rubbish, which includes lots of polythene and even some bubble-wrap; also wooden pallets.
Other tile shops may have similar to get rid of. Someone else suggested bed shops, for the polythene wrapping that mattresses come in. I did ask at Focus today if they had any polythene wrapping going to waste - they said no.
I have a good supply of plastic for my home-made cloches now, thanks to Topps Tiles. :)
