4th Singularity London: Southwark free quest is the best AP & time-saving location to farm Homunculus Baby. You'll get about 3x Homunculus Baby by spending 190AP. Evil Bone is also dropped.
Here is a list of servants that need Homunculus Baby to perform Ascension and Skill Up.
Rashoumon event (Ibaraki Douji event)
Murder at the Kogetsukan
Inheritance of Glory
5 Star Servants Tier List: Best Servants [5/9 updated!]
Best Craft Essence: Craft Essence Tier List
Best Servants [★4]: 4 Star Tier List
Caster of Midrash (Queen of Sheba)
Free Servant Guide
Jeanne d’Arc (Alter)
Piece of Year 2030 (A Fragment of 2030)
Divine Demon-Poison Sake