When it comes to describing something that is commonly done or accepted as the norm, there are a variety of synonyms that can be used in place of "be customary." For example, you could say that something is "usual," "typical," or "traditional." Alternatively, you might describe something as "conventional," "standard," or "routine." These words all convey a sense of something being expected or familiar, and can be useful when describing anything from social customs to business practices or cultural traditions. Other synonyms for "be customary" might include "prevail," "exist," or "occur," all of which suggest a prevalence or widespread acceptance of a given behavior or practice.