I usually try a few.
Keg, kegs, corny, cornelius, kegging, home brew, homebrew, home brewing, homebrewing, soda syrup container, soda syrup can, ball lock, pin lock.
Sadly, people in my area seem to be holding on to them pretty tightly, for the most part.
I feel like the entire homebrew community is looking for that magical list of 22 corny kegs for 50 cents. There are plenty available online, go buy them.
How patient are you? Fantastic deals are rare and sell immediately but if you wait long enough you may get something, especially if you setup an automated search/notification. Most usually you'll find folks "selling" illegal kegs for $50-$75. Cornies are much harder to come by then sankeys, if that's what you're looking for then i suggest just going out and buying them. If you need a quantity here's a good source: http://www.mceverdistrib.com/syrup-tanks
Check out ifttt.com you can set up a "recipe" as they call it to search for different keywords in any ad posted. Then you can set it up to email you or text you a link to the ad.
I've got a couple set up and have come across some decent deals.