I voted "other" because I don't actually have any "method." I don't find books so much as they seem to find me.
Many of the books I read are ones I discovered while looking for other books. I might start out from any one of the sources listed in the poll, but wind up using another source when I actually find a book.
A review in a magazine might send me to the used bookstore, where I don't find that book, but I do see another book which interests me but I check the reviews on Amazon and see that the author has another book which interests me more, so I buy that one. Often, one book just leads to another, whether by author or topic.