Traditionally published books:
Available in hard copy or eBook forms direct from the publisher, or from retailers like amazon.com, barnesandnoble.com, kobo.com and iBooks.
 
Independently published material:
Coloring books are available in paperback from retailers like amazon.com, barnesandnoble.com, and tower.com.
 
International booksellers that carry some or all titles:
Canada: amazon.ca
United Kingdom: amazon.co.uk
Australia: fishpond.com.au
New Zealand: fishpond.co.nz
Sweden: bokus.com