I must admit that I go to these types of stores everyday and the chances of my finding some of these items is greater the more I shop at them. Just because when you shop at these places you do not find the same amounts as I do, do not give up. It is worth your time to continue to think of these places as a source for cheap survival gear. Of course if you are not near a military installation you will not have the same types of items I have listed but you will still have generic camping gear available to you no matter where you live.
Now I have purchased firearms at yard sales before but seldom do they sell them at these sales. Most of the time when they do sell firearms they want too much $ or the weapons are not what I would want for my inventory. Shotgun ammo is easy to find in most places and the chances of a sale having your type of shot shells is better than you finding some .44 mag or .500 S&W.
I have even had luck at the local antique stores. I picked up a kevlar helmet and several pairs of gauntlet winter gloves at a favorite store of mine.
So the best advice I can give is to shop often and early at the stores and the yard sales. You never know what you will find and how much you might save in the long run.