The old Tamron Adaptalls can be quite good and you might be able to find one with an undesirable mount for a good price and pickup a Pentax mount for it.
Buying old stuff comes down to luck, a majority of the lenses listed in my signature have been found at thrift stores, yard sales and flea markets. I've passed up tons more because either the price was too high or because I didn't need yet another 50mm or low quality tele-zoom. Some I got luckier on than others, but all were found for much lower than average eBay or forum prices.
Last edited by elliott; 08-21-2012 at 05:10 PM.