Things have moved on since most of us were young,. Sure there are still wax crayons, but now there are all kinds of pens, pencils, pastels, paints and masses of different brushes and applicators. We can have fun like we never had before.
If you want to buy an adult coloring book, you can buy them at book and craft stores and even local art supply shops. They come in a range of sizes so you can slip one in your pocket and color during your lunch break or on the commute home. The large format books are great to invite your kids, grand-kids or even your best friend to join in the fun.
One artist that has produced some of these books is Alisa Burke and she often gives away free pages.
Here is one to get you started
Here is another
If you're an artist or illustrator you could create your own book quite easily. Just select about a dozen original works and use one of a hundred low or high tech ways to make line drawings from them.
Create an eye catching color cover. Ask your local printer to print two up back and front on 11" x 17" sheets for larger format and voila! This is a great project for your local gallery or school to raise funds. Using local content makes it even more perfect.