The home thread includes a “where to purchase” section for the book. 
Amazon is what I use, since I have a Kindle, but it has some extra hoops to get set up. Bookwalker is super popular for digital, and easy to use on a PC.
If you want to use Amazon, the explanation on how to do it is posted here:
https://community.wanikani.com/t/buying-digital-japanese-books/10603/151
For physical, CDJapan is usually the better option over Amazon, I feel, but you should usually buy anything physical in bulk orders because shipping is roughly the same whether you buy one book or fifteen, so it’s more cost-effective to purchase multiple things at once.