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
FGO Reroll Tier List [5/11 updated!]
Best Craft Essence: Craft Essence Tier List
Ryougi Shiki (Assassin)
How to farm EXP cards & level fast
Fate Grand Order Friend Request Forum
How to build an Arts team
Black Holy Grail (The Black Grail)