SMC Pentax 28mm ƒ3.5 - IMHO excellent lens
P-KA Sigma Mini-Wide II 28mm ƒ2.8 Macro - also very, very good lens with A-contacts that allows full metering modes and aperture control and it is very affordable.
SMC Pentax-A 28mm ƒ2 - Excellent!!! (expensive), also with A-contacts as above
SMC Pentax 28mm ƒ2 - the crown jewel but expensive and rare
The next recommendation is very close to desired focal length but very worthy contender...
SMC Pentax 30mm ƒ2.8
You can check the reviews for those lenses
here EDIT: I see that the OP got the lens already and posted while I was typing (got interrupted with phone call, while typing)