Have you tried here:
http://www.pdacardandcraft.co.uk/merchant.ihtml?step=2&cid=2
Bespoke Silk Florist ~ Venue Dresser ~ Chair Cover Hire ~
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Are any of these any good?
http://www.pdacardandcraft.co.uk/merchant.ihtml?step=2&cid=16
This is the right size, but not sure how thick it is....
http://www.pdacardandcraft.co.uk/merchant.ihtml?step=4&lastcatid=16&pid=104