If you are talking school shootings, they usually either loot daddy's unlocked gun cabinet with all the ammunition stored handily near by (or daddy has given them a herd of guns starting when they are about six) or they buy them from friends, crooks, gun shows, etc.

If you are talking about the toddlers shooting their parents, generally they fish them out of mommy's purse or find them laying around the house with the full ashtrays and empty beer cans.