Stationary and stationery are confused probably more by their similar spellings than by their definitions, which are quite different. The one-letter spelling difference in these two words makes them easy to confuse.
Stationary with an “a” means “not moving”: The dog lay stationary in the hot sun.
Stationery (with an “e”) refers to writing materials, usually paper.
The way I remember the difference between the two is by associating the “e” in stationery with envelopes, because envelopes are often used with writing materials.