Thief items

For the lib, you need to search for the item you want and it will show the mobs that drop it.

For pots, several recommendations:
-Grilled cheese from NLC (or an FM pot shop like 5-13) will be your most effective potion for awhile
-Scrolls you get from gachapon might be worth a lot, so you can check a price estimate at https://owlrepo.com/
-At lv 30, an easy quest opens up to get you a shoes for jump 30%, worth over 5m. https://bbb.hidden-street.net/quest/world-tour/maplestory-basic-knowledge

You may consider also joining an active guild that can help you with other questions along your maple journey. Good luck!

Edit: also the current event has a lot of goodies (including pots) you can get from raffle tickets. Talk to Cassandra in most towns to go to the event map.