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 Servants [★4]: 4 Star Tier List
Ryougi Shiki (Saber)
The Imaginary Element
Marie Antoinette (Rider)
Talon of Chaos (Claw of Chaos)
Cu Chulainn (Lancer)
Zhuge Liang (Lord El-Melloi II, Waver)