You can find your paint code on the Service Parts Indentification sticker. I believe it is inside the glove box.
It will look something like this sample:

On mine, the last line of the codes is BC/CC U 405Y.
I think it translates to Base Coat/Clear Coat Upper Siren Red. 405Y is the paint code for my Siren Red.
I found the GM touchup paint on Amazon (actually it is ACDelco) in a Pen/Brush combo by just searching for 405Y.