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I believe they are found on your my purchases page. You might have to click the "download" button in order to open up the game information. None of my purchases here have keys, so I don't know this for certain.

If the purchase was part of Groupees Build a Greenlight Bundle 44, the key will be found on that site not this one. If you got an itch.io key for the game via trade, ask the person to give you their Steam key as well.