タオルを忘れてしまいました。購入できますか？ — I forgot my towel. Can I buy one? (Public · Bath · Onsen, Formal — to staff, JLPT N4)

Gesture & etiquette

Point to yourself, then make a 'forgot' gesture (slapping forehead gently), then a 'buying' gesture.

Do not wear swimwear in most Japanese public baths. Check for tattoo restrictions before entering.