You can call the marine supply stores around town and ask if they have it. You can usually buy a roll of it that will cover all your runners. Basspro carries it.
The carpet you buy at Lowes or Home Depot will work just as good too.
Tip: When you do this, wrap the carpet across the top and down both sides of the runners, but ( don't wrap it underneath. ) If you do this the carpet that is wrapped
underneath will hold water and cause the runners to rot quicker. A very good way to secure the carpet down both sides is with stainless steel staples in one of
those big staple guns like the big chrome Swingline guns. You can get the staples at Lowes or Home Depot.
My .02