The books are available at the following online websites:
Clicking on a name will take you to their website. You may need to search to find the the title you want and the best prices.
Most of the sites offer e-books, if you're looking for a paperback or hardback, the best option is
Amazon-Kindle.
Amazon-Kindle (See the "Titles Available" page for links to the relevant Amazon page)
Tolino (Thalia)