There were some adult coloring books at Wal-Mart in the Magazine section. I thought they were too simple and wouldn't be challenging enough to add shading, etc. I really think the ones at JoAnn craft stores are better. The paper quality is better and would lend itself to watercolor pencils without the paper shriveling to get those werd wrinkles.
I am a digital illustrator. I love COLOR, and for me, real life on my desk coloring books help with my OCD and keeping things in order, coloring in the lines, holding the pencils or crayons, putting them in order by size, I love the entire process.
I am a fan of adult coloring books and coloring pages. Ben Kwok has a Facebook page entitled Ornation Creation. He allows free downloads of his templates. They are amazing! Print them on mixed media or bristol paper depending what you chose for art materials. Super fun to work with these.