The Angle Finder 2 listed on the KEH site specifically states it is for the older Mamiya cameras - the eyepiece fitting on the prisms for the Super, Pro and ProTL are different and this finder won't fit. The only right angle finder you can use is the Angle Finder N. The fitting for the latest angle finders, designed for the Mamiya AFD and DM 645s look different and I fear they're not compatible either.
And you are right - they're pretty rare and were not made in great numbers because it was assumed that most people would have had a waist level finder which carries out pretty much the same function but without the benefit of in-built dioptric adjustment. Of course, with the WLF you won't have the benefit of AE metering provided by 2 of the 3 prism finders, so you'd need an external light meter. The small numbers made combined with their usefulness with AE Prism Finders means that these dont tend to change hands very often. KEH is usually a good source for such accessories, as is ebay, but they can still be found at retailers around the world - a deep multi-language web search will usually reveal some.