stationary, stationery

Stationary or stationary means standing still or not moving--
People once thought the Earth was stationary.
Is the object in motion or stationary?

Stationery or stationery means writing paper for writing letters--
They gave her a box of pretty stationery for a going-away present, so she'd remember to write.
The paper-goods store has some nice stationery.
When they got married, they ordered monogrammed stationery.

Now that you know that, you can say--
"There is a large stationary sign outside the stationery store."

Note--a "stationer" sells stationery.
A stationer's is a business that sells stationery.