supercoloring,It has quite a few dragon coloring pages,from really detailed to simple & HTTYD,Wings of Fire & Dragon Tales

Homemade-gifts-made-easy,It just has this page full of coloring pages for all age groups

Wonder-day,This site has a small collection of dragon coloring pages but does feature a lot of HTTYD coloring pages

SchoolofDragons,the official website for HTTYD's School of Dragons has quite a few free coloring pages