In Fallout 76, several enemies carry caps, and eliminating them can be a good way to earn caps2. Some notable examples include:
Super Mutants: You can find Super Mutants in West Virginia. Eliminating Super Mutants can earn you 20-30 caps along with supplies2. A Super Mutant town is located east of Camp McClintock, and another great spot is in the Eastern Regional Penitentiary2.
Mega Sloth: The Mega Sloth carries caps1.
Grafton Monster: The Grafton Monster can have up to 30 caps on them1.
Behemoth: Behemoths carry a good amount of caps1.