You won't find ready-made little windows at a regular lumber yard or building supply store. Try a place that builds or sells children's playhouses. Pretty much anything you might need for a full-scale house is available for the backyard mini mansion - even Palladian windows!
Added later - I also searched shed windows - and found all sorts of smaller windows - down to 14" - even some specifically designed for coops. I was just curious, but now I'm rethinking my planned upgrade!