These are two homophones that are frequently confused. Stationary means something that’s in place, rooted, like a tree. Stationery is fine writing paper, the kind used to write important letters. (I have seen sellers of stationery spell it with an A, which always surprises me.) Here’s a simple way to remember the difference between them:
stationary has an A, as does the word stay.
stationery has an E, as does the word write.