I want to buy my clothes upstairs

Is it correct to say

1) I want to buy my clothes upstairs.
2) I want to buy my clothes upstair.

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

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 steep and tortuous stair -- H.S.Morrison> -- often used in plural but sing. or plural in constr. <a narrow private stairs to connect the upper and lower rooms -- Lewis Mumford> <lurked at the foot of one stairs -- New Yorker> <ascended a stairs -- Scott Fitzgerald>

Merriam-Webster Unabridged
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and even if it doesn't, it's of course poetical usage.

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 another reason.

(Also, as we've said in this forum before, song lyrics and poems don't always follow standard English. "Who lived up the stair" is poetic. If you want to refer to your neighbor on the floor above you in standard English, say "The lady who lives upstairs.")