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
Best Servants [★4]: 4 Star Tier List
Device Transfer Guide
How to farm Pieces & Monuments
Best Craft Essence: Craft Essence Tier List
How to farm EXP cards & level fast
Arc 2: Cosmos in the Lostbelt
Saint Quartz Farming Guide
Black Holy Grail (The Black Grail)