Many are inexpensive and good, some are legendary; Of course some are expensive and rare (but all usable in a 21st century camera with on-body shake reduction to boot). Usage of old lenses are well supported by the camera bodies with focus confirmation and exposure metering. No other camera provides this sort of direct backward compatibility short of paying for the most high end camera bodies.
The newest lenses now are the DA series. You should not be lacking in any focal length especially once you add in the 3rd party options.
You can have a look here for the current lens options and prices.
Just note that the list excludes the older F and FA lenses which are AF and auto aperture as well as even older manual lenses (to form a very long list indeed!).
Older lenses are not too hard to obtain on the internet or in BnS here.
Do browse about in the Pentax sub-forum here to gather more info.