They can be purchased from Pepin, only in single player, after a character ventures down to Hell (dungeon level 13) or enters the Hive or Crypt. Pepin will not offer elixirs that boost a stat that you've already maxed out. Elixirs of Strength can also be bought from Adria in single and multiplayer, but they appear on her items list randomly and infrequently. She only offers elixirs to players who have reached level 26 in multiplayer, or who have entered hell, the Hive, or the Crypt (in that game) in single player. Elixirs can be bought at a cost of 5000 gold. They can also be dropped by monsters and chests, in the lower levels of the dungeons.