There were four Minolta bellows lenses in the last series (Rokkor-X) and an earlier 100/4 with a different optical formula. The 12.5 and 25 are seldom seen. I bought the 12.5 years ago at a camera show for $50. The seller probably did not know what it was used for. The 100 came from an ebay seller. The 50 came from a store in Metuchen, NJ and the 25 was purchased directly from Minolta when they had their headquarters in Ramsey NJ. There were two on display. After making sure no dealers had one and after reading my letter requesting an exception to their policy, they sold me the lens for a steep price. If you are not a collector then I suggest you consider some other "micro" lenses as possibilities too. Canon made 20mm and 35mm lenses. They are in RMS mount and would require adapters for use with popular camera mounts. Olympus made 20mm and 38mm lenses in RMS mount and than made different versions in these same focal lengths but in OM mount and with auto diaphragms. In addition to these, Olympus made 80mm and 135mm bellows macro lenses in OM mount. They are optimized for different magnifications and are supposed to be very good. Nikon made a set of Macro Nikkors for the Multiphot in 19mm, 35mm, 65mm and 120mm focal lengths. The 19 amd 35, if I remember correctly, are in RMS mount, the 120 is in Leica 39mm mount and I don't remember what mount the 65 has. I think it's also Leica 39mm. Many of the things you can do with the 12.5 and the 25 can be done with reversed wide angles, reversed cine lenses and enlarging lenses. I recommend a used copy of The Manual Of Close-Up Photography by Lester Lefkowitz. These can easily be found on internet websites specializing in used books. The book explains all of these possibilities and is an excellent reference for close-up and macro photography, however these are defined.
The usual sources for finding this equipment include eBay, KEH, B&H, Adorama, Woodmere camera, Henry's, Central Camera, Midwest Photo Exchange and some other advertisers in Shutterbug and Popular Photography.