Well, I can't help you as far as where to buy inexpensive defense rounds, since I shoot .22 all I really buy is the ten dollar bricks.

You might want to worry about over penetration. If your neighbors are nearby you don't want rounds passing through the walls and hitting your neighbor's kids. Just about any ball ammo will go through walls. Most walls are just a two layers of sheet rock or siding and maybe insulation. I have personal experiance with 9mm ball going through exterior walls, and I've heard stories about even .22 going through interior walls (as much as people poopoo .22). So you might want to consider finding something that your weapons will shoot that won't go through the walls. Expansion may cover this (I'm not familiar enough to say for sure), and they make special rounds that are essentially tungsten powder that fragment on impact and won't penetrate walls. Especially important in the city, but even in the country you don't want bullets to go into your kid's rooms. Thats my little piece of advice. I'll let others debate the subtleties of stockpiling ammo. Most people who own guns do some level of stockpiling if only so they can avoid running to the store on the way to the range.