ADAM is found in Little Sisters. You can get it either by Harvesting or Rescuing them. Rescuing them gives you less ADAM in the short term, although Tenenbaum will give you cool stuff like extra Plasmids, rare ammo types and other rare items. Every three rescued sisters gives 440 ADAM plus some stuff. Harvesting the Little Sister will give you more ADAM, but Tenenbaum will definitely not thank you for killing the girld. Every three harvested sisters gives 480 ADAM, but no additional perks.