As far as I can tell, the mid - late sixties style style doesn't exist. I had better luck with the '69 - seventies Fender style grill cloth. Look at the 2000S pics in the "Sunn Museum" and you'll get a pretty good idea of the difference between the two.
The closest I've found so far is from Wendell Fabrics Corporation;
www.wendellfabrics.com The style is #SF7005. The rectangles are the same width but slightly deeper vertically, and there is a little less black thread in the vertical warp. Pretty close though. The smallest piece you can get is 72" X 72" and it's $50 -$60.
