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.
5 Star Servants Tier List: Best Servants [5★]
Best Servants [★4]: 4 Star Tier List
1-3 Star Servants Tier List
Valentine event: Chocolate Lady's Commotion [US 2018]
Nero Claudius (Bride)
FGO Reroll Tier List
Jack the Ripper
Talk on the Heated Sands
Street Choco Maid
Kitchen ☆ Patissiere
Holy Night Supper