Try a marine supply store.
Boats US, West Marine or a local Marina supply store all stock this material.
If you don’t have one on your area, try Boats Us or West Marine on the web.
They have tons of stuff like this at fair prices.
You may also try motorcycle shops or mail order houses.
Other sources include Pegasus Racing Supply, Skybolt, and Aircraft Spruce and Speciality.
If you are in the Seattle area, Boeing Surplus (yes the aircraft Manf.) has a huge warehouse and sell all kinds of things to the public.
I have seen this item there as well as everything else you imagine.