Here’s a list of all the pages I could find so far that offer free, printable mandalas for your coloring enjoyment. If you see a link here that you don’t think belongs, please contact me and I’ll remove it. And if there a page not listed here that you think should be, also please contact me. The main downside to printing out these free mandalas for coloring is quality. The images generally aren’t terribly sharp. But, that may be OK for what you want to do in the moment… just have a mandala on a piece of paper in front of you and zone out while you color it… no judgement, no worries.
The one exception in this list is Coloring Castle. They have a handful of very sharp designs.
Art-is-Fun.com offers a downloadable e-book of high-quality mandalas (for a fee).
In French, with only 6 mandalas that can be printed for free: http://www.mandalarbre.com/volume_1.htm