In reply to Timmd:
A couple of companies sell fabrics through mail order, Point North are good, and will give you samples and information about the best uses of the fabrics:
http://www.pointnorth.co.uk - 01407 760195
Pennine Outdoor also are similar:
http://www.pennineoutdoor.co.uk - 01484 689101
Although I wouldn't imagine a natural fabric would be too suited to this use, as to make it durable and water resistant enough would mean it would be quite heavy.
I am baffled as to why you may want it to rot, are you trying to make a biodegradable bouldering mat?