eBay
Here's a nice piece for the fan of vintage Pentax glass.

This is a 28mm f2.8 from the Pentax A  series, made in the 1980's and introduced in the Super Program era. However, as it has the electric contacts on the mount, it is nicely compatible with current Pentax digital SLRs, especially as a compact wide angle on the full-frame K-1. The contacts allow for aperture communication and use of program or shutter priority modes, and manual modes without having to use the green button.

This lens is cosmetically very clean, with only minor signs of handling and gentle use - well cared for.  The five-blade aperture is clean and snappy. All glass surfaces are clean and free of haze, dust, scratches, or fungus. Focusing is smooth and precise from infinity to 30cm (1ft)

Included are the original front and rear caps. The front filter size is 49mm.