http://thegaugeshop.com/shop/page/3
I found this website last year, when i changed the engine in my mk1 i decided to opt for negative earth. He converted my older style face Smiths RVI tach, probably replacing with RVC internals, so that it would work with electronic ignition and negative earth.
One comment i can make, at the time he was very hard to get hold of! However, it was quick and the item did look brand spanking new once returned.
His name is Alan Davis, and he is based in Edenbridge, Kent. (If that is of any use to you, or nearby!)
Thom