It is well-reported that America is a land of 312 million people and somewhere between 310 million and 320 million guns. (There is no firm figure, because thankfully only a small fraction of Americans live in locales with gun registration.) Of those, there are about 80 million handguns in circulation. And of that 80 million, I would venture an educated guess that there are less than 50 million holsters, to match. This is because most handgun owners are not regular handgun carriers. The most lopsided "gun-to-holster" ratios are with .22 rimfire handguns, and large-frame, long-barreled revolvers. I suspect that perhaps only 25% of those handguns have an accompanying holster. There are also more rifles and far more shotguns out there than there are carrying slings for them. (I'd roughly estimate that less than 10% of shotguns have slings.)
These disparities represent a huge opportunity for a post-collapse cottage industry. In a post-collapse world, suddenly almost everyone will want to be armed at all times, and they will be eager to barter to fill those needs.
Get some practice at holster and sling making. Then stock up heavily on leatherworking tools and supplies, tanned cow hides, sheets of brown or olive green Kydex, rolls of brown or olive green nylon webbing (for slings and straps) sewing awls, waxed nylon thread, rivets, snaps, sling swivels, and buckles of various sizes.
Also keep in mind that because of its length and padding, the venerable U.S. military M60 sling is one of the most versatile slings for re-purposing. They can be used with a huge variety of rifles and shotguns. So if you don't have craft skills, then you can at least buy a pile of those slings to keep on hand for barter. (They are quickly and easily shortened, with a snip of scissors.)
I should also mention that nearly any handgun with a positive external safety lever can be safely carried in a Nalgene water bottle pouch. (Warning: Glocks and other "safety in the trigger"-type pistols can only be carried safely in specifically-made holsters that fully enclose the triggerguard!) Yes, these pouches are bulky and slow to access as a makeshift holster, but they will fit about 80% of handguns. But their bulk also camouflages a pistol--since they don't look like a holster. That can have advantages in some situations. If it the pouch is too deep, then just add some balled-up pairs of spare socks, or some Israeli battle dressings, or a couple of folded bandanas. And by the way, the same pouches also work reasonably well for carrying shotgun shells and many types of magazines.
Someday, you may be very glad that you stocked up. - J.W.R.