Remember to return the first boss head to the altar where you start els im not 100% sure if you can advance.
But els boss 2 is in the black forrest and if i recall correctly you find his location in the crypt with skeletons. Basicly every time you find a skeleton in the black forrest area you know there is a crypt somewhere nearby. ;-)

Boss 3 ( bonemass ) i found the location of him after visiting the swamp area and in a stone tower there was a stone with his location. ( els i think it can be found in the submerged crypt in the swamp.

ps. when you put the head on the altar remember afterwards to click the head and you get a special time limited ability that can help you with example cutting down trees faster.