[Brett Riedlinger]

Found this local company in the Akron, Oh area that makes natural rubber tubing. So I called to place an order and they said to contact McMaster Carr, they sell their product. Here's a few links.
http://www.primelineindustries.com/
http://www.mcmaster.com/ search for "LATEX RUBBER TUBING" at McMaster Carr.
My friend replaced the tubing on his GUs with this product, and said it made a huge difference on the marks.
Thought some might be interested.

 

[eva b.]

which size of the tubing did your friend get?

 

[Brett Riedlinger]

I'll have to double check but I believe the #5234K43 7/16" OD 1.03/ per ft. This tubing fits the Zinger's pulleys better then the GU's, but under a little tension/stretch it's fine.