Hi Sirius Glass,
The Zeiss Mutar T* 2x is a dream to work with, you will find it used any day on eBay.com. I bought mine used, as well as the newer Hblad FE Tele Extender 2x (made by Fujinon), which I got really cheap, because it had stains on the outside of the barrel (I will cover this up with a spare rubber ring from the 4/180mm Sonnar). I would personally avoid Kenko and the like from Japan to be used in conjunction with the superb German Carl Zeiss T* optics.
The tests I have made (Fuji 100 PROVIA 100F, Fuji ACROS 100, Kodak T-Max 100 and Ilford FP4+ 125 ISO) look just as good with either extender, although I prefer the Mutar with it's Zeiss T* multicoating (Tarnung) for color work. Very high microcontrast. I have found that the best possible match is the superbly sharp Sonnar CF 4/180mm T* telephoto lens, which yields a long 8/360mm lens of extremely high resolution. I have also tried it on my Tele-Tessar CF 5,6/350mm T* lens (which produces a 700mm lens) and on the Apo-Teletessar CF 8/500mm T* lens (which produces a 1000mm lens).
It goes without saying that a very sturdy wooden or metal tripod and solid pan or ball head is needed.