How do you do it? Is it done by doing the winterhold college quests, or is it done by leveling up the respective skill?

I.E.

'Destruction Adept spells are available from merchants once you hit 40 Destruction skill'

or.

'Destruction Adept spells are available from merchants once you get to etc/etc in Winterhold College quest'

Does anyone know?

Buy the books in the stores from people who sell spell books.

Thats not why im asking. Those people that sell spell books only have novice/apprentice level spells. Im asking where to get Adept/Expert/Master level spells.

They don't become available to purchase until you've reached the appropriate level in whatever magic you're using.