1. Any bookstore can order a copy of Walker's Key for you through IngramSpark -- please consider supporting your local independent bookstore.
2. Several bookstores have been stocking the book and will probably have a copy available immediately for you to pick up if you are nearby. These stores include: Phoenix Books, Burlington, VT; Parnassus Book Service, Yarmouth Port, MA; Reed Books, Harwich Port, MA; Provincetown Bookshop, Provincetown, MA; Tombolo Books, St. Petersburg, FL.
3. Tombolo Books in St. Petersburg, FL, would be very happy to send a copy of the book for you. Here is the link: https://tombolobooks.com/?searchtype=keyword&qs=9781949066234&qs_file=&q=h.tviewer&using_sb=status&qsb=keyword
4. You can always purchase the book through Amazon, though I hope you'll first consider supporting an independent bookstore: https://www.amazon.com/Walkers-Key-Frank-Haddleton/dp/1949066231/ref=sr_1_1?keywords=Haddleton&qid=1558180002&s=books&sr=1-1
5. An ebook version is also available.