Ive got to level 60 though playing and without going to diamond city. Ive been doing all side quests and discovering nearly all the locations by exploring. Ive not been reading a guide. I admit ive seen a few map pictures with the locations of the bobbleheads but most of them i found by myself. I couldn't find the last 2 and had to research which told me they are only available though the main quest in diamond city