Where can I get a golem's heart?
All golem type enemies have a chance to drop Golem's Heart, although I personally have only found hearts in earth-type golems, but I'm not enough of a sample size.
In general I would suggest that before you encounter monsters that have rare drops needed for alchemy (Ekkimara, Nekker Warrior, Cave Troll, Cockatrice, Basilisk, and so on) you save your game and reload if you don't get the alchemy drop.