A few DM's try to limit the use of banishment by making players understand what their adversaries dislike and fear, and carry that information in their possession when applying the spell. Some simply state that a pouch with components contains the items you'll need to ward off the majority of your adversaries.  It's all dependent on the discretion of your DM and how much you'll use to expel enemies during the campaign. If you're only using it in the instances it makes sense to use it, most DM's assume you've got the components required.  If, however, you're making use of the spell frequently and removing your opponent before each fight and every battle, they may make it more clear what you should carry with you to expel your foes. Additionally, although banishment can be fun, many other fourth-level spells can be utilized and you should try branching out!