For starters, you might check out the Lens Review database on this site to see which A lenses are getting high marks from users. You could also go to the Pentax Photo Gallery and search the pics based on A lenses that you're interested in just to see what they're capable of producing. The good news is that it would almost be easier to list Pentax lenses to avoid rather than which ones to seek out because most Pentax glass is pretty good. I don't know how it is in Canada, but here in Oklahoma, pricing can vary wildly depending on where you find a particular lens. I'm still kicking myself for passing on a 135mm f2.8 that I saw in a junk shop for $10 this past spring.
Garage sales, junk shops, pawnshops, and the back shelves of your local camera stores are good places to start looking. Good luck!