At the risk of being obvious, have you tried Amazon? Also, if you know the publisher, you can order it directly from them, either yourself or through a book store. Barnes and Noble, for example, can order anything in print, even if they don't ordinarily carry it. If it is out of print, there are plenty of used bookstores or out of print search services.
I am a devotee of St. Pio myself. I have heard of that book, but never read it.