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!]
Saint Quartz Farming Guide
How to build an Arts team
Demon (Demonic) Servants & Enemies
Best Craft Essence: Craft Essence Tier List
Zhuge Liang (Lord El-Melloi II, Waver)
Jeanne d’Arc (Alter)
Cu Chulainn (Alter, Berserker)
Jack the Ripper
Beyond the Wish