Numerous new and used copies of A Life in Aikido
are available (find them at bookfinder,com), but as the book is currently out of print, the prices are steep. Kodansha USA has plans to reprint the book, but the schedule is indeterminate. Your best bet to get one at a reasonable price is to watch for it on eBay.
One way to get a complete listing of those books available in Kindle format is to go to http://bibliography.aikidojournal.com/
and use the search function (it's the green magnifying glass button) to look for "Kindle". You'll then be able to scroll through 12 pages or so of listings, each of which will contain the corresponding ASIN number. You can do the same for Barnes & Noble's e-books by using "Nook" in your search.
A note about Kindle (and other e-book format) titles: Occasionally you may find that an e-book title is shown to be "not available for purchase". This most often occurs when the agreement between the author and the seller has expired, or has a finite number of resale permissions which has been reached. This is usually a temporary situation.