Under the rules for the 100% Category, it says stabilizers and steroids. Does this mean just buying one of each from the cages, or finding all available in the game (i.e. Treasure Photo, Toy axe shadow puzzle, etc.)??

As stated in rules "all collectibles"

Every single: Mr Everywhere Antique coin File Stabilizer Steroid

That means, get every bobblehead, pick up every file, get every coin and get everything out of the cages necessary, find all hidden steroids and stabilizers. That means, the ones hidden in the fireplace, under the bathtub near the end, the one in the treasure chest via toy axe; as far as im aware treasure photos arent necessary to get collectible items (they give you ammo and a repair kit on easy, apart from the first one that gets you a steroid) but they MAY count as files, so I recommend going to the leaderboards and watching Carcinogen's 100% run to double check.