I've said this before but it's worth mentioning again. When I decided to shoot vintage guns with short chambers and requiring low pressure levels, I quickly realized these kinds of shells were not always readily available. And seeing as I don't reload, I needed to come up with a solution to that problem. Now, I only occasionally shoot clays or waterfowl. The vast majority of my shooting is upland hunting. So I started buying ammo when I saw it in the right size and load, especially at what seemed a good price. I did it over time so the financial burden was mitigated somewhat. But I now have 1000s of rounds in whatever gauge, load and shotsize I might decide I want to use. I could easily go 5 years without buying anything and not be short. I shoot 4 different gauges and the only one I'm light on is 28 because that is a relatively new gauge for me.
The only solution to the pressure/short chamber issue is inventory.