There is no trick involved. Worst case scenario, a little detective work is all. The answers you seek are usually the most obvious in a very obvious lot.
I'd say most all of the Baen novels can be had off of Amazon.com or the like. Some more uncommon ones, like Heart of the Tiger or Action Stations, you can grab at Half.com for a very cheap price (I got a mint copy of HotT there for $1.25 no more than 3 months ago and Action Stations for a dollar under it's cover price in the same condition).
If you're looking for End Run, Half.com seems to have a good ammount, but since I mentioned that Half.com has them in good quantity, the price on it has shot up to stupid prices, like $40 for a dog-eared, spine worn copy.
If you're looking for a strategy guide, I'd suggest hitting not only Half.com and the devil that is nessessary - Ebay - but video game sites. Sometimes they have stuff kicking around. Also, call the printing company or email them, it can do wonders.
I was looking for "The Prisoner, vol 3" soundtrack, which no one had. Guess who had it? The people that printed it. Wow, big surprise, eh?