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.
]Journey to the West Event Re-run
Hunting Quest Part 1
Fate/Accel Zero Order Advance Campaign
Fate/Accel Zero Order Event
Lostbelt No.1: Anastasia
5 Star Servants Tier List: Best Servants [4/13 updated!]
Best Servants [★4]: 4 Star Tier List
1-3 Star Servants Tier List
Xuanzang Event: Journey to the West Re-Run [JP 2018]
Anastasia Nikolaevna Romanova
Irisviel (Dress of Heaven)
Ramesses II (Ozymandias)
Black Holy Grail (The Black Grail)
Talk on the Heated Sands
Nine-Headed Bull Demon King
Lunar Hot Springs