Before the .223 became popular, the .222 WAS popular. In fact, the .222 was the basis for the engineers that designed the first AR-15's before the US Army called them M-16's, but they wanted to make it ever so slightly different. But basically it's NEARLY but not quite the same cartridge. You can use the same projectiles for both cartridges if you reload your own...but sadly they won't or shouldn't be swapped in the wrong chamber.
Today, I'm guessing that finding boxes of .222 won't be all that plentiful, but again, if you roll your own, getting cases should be easy, and finding powders and projectiles woud be available anywhere you can buy reloading supplies.