Significantly rarer than the R128 version AFAIK (JK would be the definitive source of course!) IIRC, John released a Radeon variant shortly before he stopped supporting the ViewFinder - so logically there shouldn't be many of these around at all!
Each card (Mach64/R128/Radeon) works totally differently to the others, and unhelpfully many different varieties of the same card (eg. different manufacturers of the Rage 128) exist and behave differently too. This is probably why the ROL driver supports only a subset of cards (most R128s) - development time for three drivers and their different card varieties becomes quite prohibitive.
Mach64 support is pointless - a quick trip to EBay or a RISC OS dealer should be able to pick you up a R128 AGP card for very little. An Ebay search just now suggests purchase prices of £5 for these cards. Caveat emptor - note that not all varieties will work, but for that price you can afford to buy an unsupported variety once or twice; RISC OS dealers will no doubt charge more but at least guarantee finding a working variant.
Some people also get utterly confused as to which card is in the machine (assuming R stands for Radeon, or 'It's ATi so it must be a Radeon'), which doesn't help! Generally Mach64s had 8MB RAM, Rage 128s had 32MB RAM, and the Radeon 9200 which I think John supplied at one time has 128MB RAM. I think on the original driver *ViewFinderInfo told you more about the card.