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.
5 Star Servants Tier List
Best Craft Essence
Oda Nobunaga (Archer)
Best Servants [★4]
Zhuge Liang (Lord El-Melloi II, Waver)
Cu Chulainn (Lancer)
Piece of Year 2030 (A Fragment of 2030)
Talk on the Heated Sands
Victor from the Moon