I guess I am just beating a dead horse here, but if you can't find the rounds why would you want to use rock salt or the like as a defensive round? I just don't understand the logic. You should have the mindset that if you are a going to pull the trigger of the gun that you damn well better mean it and understand the consequences of your actions. If you have to think "should I pull the trigger" you probably should not shoot.
Couple of scenarios here.
Someone in your house. You some how sneak up on the them while they are trying to steal your computer. Are you just going to shoot without saying anything. If you said yes (then I guess I can understand the less lethal rounds) but if you said no, now you have a situation. That intruder can do one of two things ... retreat or fight. If they choose to fight, why would you want a less lethal round? They are coming to harm you. I understand that rubber bullets hurt and bean bags shock the sytem, but you are not going to "knock the guy out" as you say with those rounds.
Second scenario
Guys breaks into your house and you see him at your bedroom door. Assuming you can get to your gun (big assumption as they have the major upper hand since you were asleep, disoriented and they can charge you ... remember FBI testing says that if an attacker is at 21 feet or closer to you, you can't physically pull and gun and fire a round before they reach you if they charge you ... and that's when you are alert and in a defensive mindset) Once again, if they choose to attack you why would you want a less lethal round?
I have read several things online, several books, and have attended seminars on self defense. Understand me that I sure as hell don't want to kill anyone as I think that is something that will stick with your entire life (in more ways than one), but in a real fight for your life, you have to be willing to do some things you may not have ever given thought to. Could you bite someones nose off in a fight to save your life? Could you poke an eye out with your thumb keeping the pressure on till you thumb is embedded in the socket? Both of these scenarios could happen and if you aren't willing to do them, you could lose that fight and your life. Your attacker could easily do them to you.
That intruder or attacker has no intention of fighting fair or fighting with honor, why should you? Acting honorably to someone who wants to kill you can have severe consequences that endanger you and your family.
Sorry to rant. I hope you have put alot of thought into your preparations, and if you haven't you need to. I understand and get tired of gun forum commandos saying to shoot here and there and talk like killing someone is the equivalent to making toast ... it is not. I am not one of those guys. I used to be one of those guys that said, I would rather shoot them in the leg and wait for help. The more I have learned is that idea is most likely not going to happen and I am putting myself in more danger by thinking I can make that happen.
Everything I have read and been taught from a child learning to hunt ... if you don't plan on killing something with that shot, don't pull the trigger. Period.
Good luck in your search if you are dead set on this approach, but I must say that I don't agree with the logic stated in the OP.