Thanks to whoever tipped me off to kevinscameras.com, but the 15/3.5 lenses they have are SMC-Pentax (K-mount) not SMC-Takumar (M42). K-mount 15s aren't that hard to find since they were made in K and A versions from 1975 until 1980-something and apparently still availible by special order, and I'll bet that third-party 14/2.8s can be had in K-mount. M42 is a lot trickier since they only made 15/3.5s in M42 mount from 1974-1975 and the only new M42 cameras made since the 1970s are those retro Bessaflex thingies and oddball Russian models.
Believe it or not, somebody makes an adapter to put K-mount lenses on M42 cameras, but it's got glass in it and it's likely to really destroy the optical performance of something like a 15mm lens, even after the APS-sized sensor crops the edges off. DJE
