I think the only thing to do about insuring that you always have ammo for a gun that you have is to have guns in every common caliber. Having a 9mm automatic and having just salvaged an entire crate of .45ACP is like Alannis said, "It's like ten thousand spoons when all you need is a knife."
Keep handguns around in .45ACP, .40 S&W, 9mm, and .25ACP. Keep revolvers around in .44MAG, .357MAG (.38), and .22LR. Keep rifles around in .308, 7.62x39, 5.56 (.223), and .22LR. And of course, a 12 ga. just because. You would have to happen across a bonanza of .380 ammo to find yourself SOL for ammo.
Although, if you had moon clips for your .357, I think .380ACP would actually feed nicely. Hmmm. Though, I wouldn't try feeding it through a .38 S&W, even though they fit.
Just stay away from the "experiemental" calibers like .357SIG, .455 Webley, and you'll do fine.
As for the person who was confused over .38 S&W vs. .38 Special (SPL), they're both cartridges of comparable dimension, but the SPL falls in between S&W and .357 Magnum in case length. You can feed SPL ammo through your .357, but I wouldn't suggest .38 S&W through your .357 with any regularity, since it's 4/1000 of an inch larger, it will result in accellerated barrel wear.