Hi! Thanks for playing my little game. If you played in generator mode, cotton is spawned in your throne room. Unfortunately it degrades over time, but it should stick around for at least a few years. However, if you don't build a cotton farm prior to it vanishing completely, you are screwed. If you're playing the tutorial, cotton should spawn when after you've completed objective 6 / 7 and built a metal smelter.
You can always add cotton with the developer tools if you enable dev-mode in the launcher.