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I got mine from my local builders merchant. They'll probably sell it to you by the metre so you don't end up with far too much. I just asked for polythene sheet. They asked what colour. ;DGeoff.
I got some from Ebay, about £4 a roll. http://cgi.ebay.co.uk/WEED-CONTROL-FABRIC-8mt-x-1-5mt-LANDSCAPE-FABRIC-BLACK_W0QQitemZ310083762090QQcmdZViewItem?hash=item310083762090&_trkparms=72%3A12%7C39%3A1%7C66%3A2%7C65%3A12%7C240%3A1318&_trksid=p3286.c0.m14Havent used it yet but its supposed to be better than polythene as it lets water in so keeps the soil moist.
Do you want it to be water permeable? If so I got a big roll from Wickes for about £20 last year...