12c here looking to reclass as a 89b. What are all the duty stations for a 89b?

Literally anywhere/everywhere.

Take your handy dandy CAC Card, get a login for FMSWeb, then Do a query on all AD MTOEs, TDAs and JMDs with 89B10 and 20. Personnel recaps, not equipment.

There are 89s at Division, SBs, CSSBs in OD Ammunition Companies, and BSBs all the way Down to FSC level. There are a handful of NCO slots in SPO shops. Somwhere you see a Division. SB, CSSB, or BSB know there are slots.

Everyplace that the Army is worldwide? We all use bullets for our primary firearm and sidearm. In one way or other, every unit has one or more.