I want to buy my clothes upstairs

Odd sentence but the word is always upstairs, not upstair.

Odd sentence but the word is always upstairs, not upstair. The opposite is always downstairs, not downstair.

Another variant:

When I was twenty-one
It was a very good year
It was a very good year for city girls
Who lived up the stair
With all that perfumed hair
And it came undone
When I was twenty-one

Well, you have stair in that song, but it may relate to a multitude/series/flight of steps:

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stair

a series of steps or flights of steps connected by landings for passing from one level to another <a steep stair ... provided access to the upper floor attics -- G.E.Fussell> <climbing down the ste

Nona The BritOdd sentence but the word is always upstairs, not upstair. The opposite is always downstairs, not downstair.

Thanks, but why this is odd? And how to correct it?

I believe Nona meant it was an odd thought to express: I want to buy my clothes upstairs. Is there something wrong with the clothing sold on the first/ground floor? Are all the better brands sold upstairs? Most people don't head into a shop with the idea they want to buy their clothes on one floor or another. At least, those are my thoughs. I'm sure Nona will chime if she thought it was odd for an