Pentax K-S2 has a Pentax KAF2 lens mount and currently there are 141 native lenses available for this mount.
K-S2 also has a sensor based image stabilization system which means that all of these lenses will be stabilized when used on K-S2.
Since Pentax K-S2 has a APS-C sensor, it has a focal length multiplier of 1.5× so you have to multiply lenses original focal length with this multiplier to find the Full Frame equivalent focal length of the lens when mounted on K-S2.
You may also also be interested in these:
Pentax K-S2 Lenses with Image Stabilization
Pentax K-S2 Lenses with Weather Sealings