Its not going to be that easy to find truly "worn-out" handguns, unless you include those made out of ZAMAC. When guns break, they usually get parted out.
You have to remember that, even if a handgun is "worn out," it's still a handgun and must be treated as such during the purchase, including any applicable transfer requirements, backround checks, and fees.
To add to what Mike said - that does make it a bit more difficult or non-cost effective to get rid of them too when you simply don't want them. You can't really just throw them away.
I think I would hit the gun shows and ask about junkers. Dealers and gun smiths who have been in business for a while often have several junk guns for which parts are not available and are not worth fixing. Sometimes these have missing or broken internal parts but appear to be complete, just what you would want.
But those who know me know I always have at least one note of caution. This time I have three.
Don't advertise in the paper saying guns wanted will pay cash; I don't know a better way to call police attention to yourself because the cops will think you are out to supply criminal gangs.
Don't let casual visitors (repairmen, etc.) see your display. They won't know that the guns are inoperable and the word will get around that so-and-so has a whole bunch of guns. That may attract the attention of people who are not nearly as nice as the cops, and who may decide to come for your guns.
Don't let yourself start to think that you could fix up that old H&R or IJ, or H&A or F&W or Tramps Terror and sell it. That is the way to a money pit with no bottom, not to mention the path to the insane asylum.
I know a man who brokers guns from estates, police trade-ins and also confiscations. He has some as is stuff as low as $50.00. As mentioned, these are still firearms. If sold by a licensed dealer (as he is) or pawn, your purchase and successful transfer is subject to an NICS background check and ATF form 4473 for each firearm.
I once knew a man who got some old revolvers, had them chrome plated and displayed them on blue velvet in a sunken top coffee table. The display was spectacular, but he had another motive. If some BG broke in, he would grab the shiny (and deactivated) junk guns and not even think of looking for the home owner's real collection, which included Thompson SMG's, a couple of Pedersen rifles, a gas-trap Garand, and other juicy items.
You know, at first, I thought the OP was nuts to want old junker guns to hang on his walls. But, thinking this through a bit, this might be an excelent decoy for thieves looking to steal guns. You have workers in your house, etc. - they see your wall of "prized" (junk) guns and ignore the hidden safe stashed away in the back of an obscure closet. Maybe even include a few quality nickel plated replicas - thieves love shiney guns!
If my wife would let me do something like this - I could see myself having fun with it.
I once knew a man who got some old revolvers, had them chrome plated and displayed them on blue velvet in a sunken top coffee table. The display was spectacular, but he had another motive.
My wife has "okay" jewelry that is fairly easy for a thief to find. Her "good" stuff would be much more difficult to find. Maybe I should do the same with the guns.
I have built secret drawers for jewelry and other valuables into the toe space of cabinets. There is no reason this or a floor board compartment would not work. They can be much less conspicuous than a big safe.