The Cartoclepts can be found in the optional routes of the Deadlines and Sprial Skein sections of Scrivener's Hall. Their positions in the dungeon are randomized during each run, but there will always be 1 in both daedric realms.

Check the maps of those realms here (image 2 and 4):
https://www.reddit.com/r/elderscrollsonline/comments/11wmkf4/scriveners_hall_map/?utm_source=share&utm_medium=mweb3x&utm_name=mweb3xcss&utm_term=1&utm_content=share_button

Once you open a total of 20 chests with the keys you obtained from multiple runs with your group and defeat the optional boss in the vault near the start of the dungeon (see image 1 in the Reddit post above), you can have a chance of looting a unique Rift Treasure Map, which contains the tool used to locate the Cartoclepts in the Deadlines and Spiral Skein sections of Scrivener's Hall.