After realizing how useful firearms are, I started using them a lot more. Unfortunately, they have very limited ammo. I have not been able to figure out how to get more without leaving the Palace and coming back another day. Is there a way to get more without leaving a Palace? If so, how?
After realizing there were multiple pages of Infiltration Tools, I found that once you unlock Ace Tools, you have access to the recipe for Reserve Ammo, which works as a party ammo refill.
Alternatively, purchasing the Persona 4 Arena DLC bundle will give you 9 Evoker accessories. Equipping one to a party member will restore one bullet for them after each battle.
Other than this, the only way to get more ammo is to leave the Metaverse.