すみません、両替で100円玉が欲しいのですが。 — Excuse me, I'd like to get 100-yen coins for exchange. (Convenience · Store · Exchange, Formal — to staff, JLPT N4)

Gesture & etiquette

Hold up the bill you want to exchange, perhaps point to a vending machine if relevant.

Do not expect small shops to always be able to make change for large bills (e.g., 10,000 yen for a small purchase). It's always better to make a purchase when asking for change.