Same sound, different word
FoundationNot moving.
Writing materials.
The one with an 'e' is the one you can buy — stationEry means Envelopes. The one with an 'a' means stAnding still.
Where it bites
Both are nouns in some uses, so part of speech will not save you. Only the spelling carries the meaning.
The vehicle was ______ when the collision occurred.
stationary · it was standing still
The office ______ budget has been frozen.
stationery · paper and envelopes are meant