writing paper; stationery; notepaper
Specifically refers to sheets of paper for writing letters, not general office supplies.
She wrote a letter on beautiful writing paper.
文房具店で便箋と封筒を買った。
I bought writing paper and envelopes at the stationery store.
She wrote about her recent life on notepaper and sent it.
I bought notepaper and envelopes at the stationery store.