7th Singularity Babylonia: Observatory (Mother Sea) free quest is the best AP & time-saving location to farm Seashell of Reminiscence. You'll get about 4~5x Seashell of Reminiscence by spending 210AP.
3rd Singularity Okeanos: Archipelago (Hidden Island) free quest is the first location to unlock where you can farm Seashell of Reminiscence.
You can also farm Seashell of Reminiscence Sunday Daily Quest: Saber Traning Ground. For other item drops, check the table below.
Rashoumon event (Ibaraki Douji event)
Murder at the Kogetsukan
Inheritance of Glory
Best Servants [★4]: 4 Star Tier List
Best Craft Essence: Craft Essence Tier List
1-3 Star Servants Tier List
Zhuge Liang (Lord El-Melloi II, Waver)
Cu Chulainn (Lancer)
Mysterious Heroine X (Alter)
Buddha of Emptiness