Buy them from me
Seriously, though, it depends how much you want to roll the dice, or how much effort you want to put into it.
ebay...well, bidding on stuff by sellers who say "I don't know much about cameras" is about as risky as it gets (aside from shipping and Craigslist). But, deals can be had. But, are you ready to get a lens that may be worse than you hoped? If you have more time than money, you should look for bundles that have a bad description or bad pics. Sometimes a lens will be pictured, but won't be in the description for example.
Garage sales...80% of stuff is for kids and the possibility of finding Pentax stuff is very small. I've run across one 1980s "kit" by total chance, and I love garage sales.
Thrift stores...Goodwill is pretty much out since all the good stuff goes online. If they put something out (at least the stores near me...one guy on another forum in the Northwest US doesn't have the photo stuff filtered before it goes on the shelf
) it's broken or they think it's broken. Shutter is stuck, film wind won't advance, lens is stiff to focus, etc. But sometimes the body is bad but the lens is good. Not a very good visit : availability ratio at most thrift stores. I go to 3 stores weekly and end up with probably 50 books, glassware, or cool random stuff to every lens.
I am a huge fan of buying off forums. Most people are enthusiasts, and you really have to be a little committed to use manual lenses
It's the best price : quality ratio out there (and honestly, you can find almost anything you want online). And most people on photography forums take pride in their pictures...squinting at blurry or out of focus 100px-wide Craigslist pics and determining the lens(es) for sale is an art form!
Regardless,
patience is the key to finding the lens you want if you don't want to spend too much. Whoops, that was way too long
Edit: One more thing, if I may...I think pawn shops are extremely hit-or-miss. A Fujica X-mount camera with a 55/2.2 lens for $220 in 2008? True story! I laughed at it, out loud, since it was $200 too much.