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.
Kara no Kyoukai (The Garden of Sinners) event
Kara no Kyoukai Event Rerun
Journey to The West
Kara no Kyoukai (The Garden of sinners) Event (The Garden of Order)
5 Star Servants Tier List: Best Servants [5★]
Best Servants [★4]: 4 Star Tier List
1-3 Star Servants Tier List
Ryougi Shiki (Saber)
Ryougi Shiki (Assassin)
FGO Reroll Tier List
Jeanne d’Arc (Alter)
Holy Night Supper
Black Holy Grail (The Black Grail)