Next to my pile of books to read I have a pile of bookmarks. I have been given bookmarks that I've kept. I've made bookmarks that I've kept. I've picked up bookmarks from book stores that I've kept. And they sit next to my books. I never know when I will start a new book and need a bookmark.
I don't tear corners off random scraps of paper to make a bookmark. I try not to have random scraps of paper sitting around. I don't use the slip of paper that comes with a library book. It's too flimsy and hard to find in a book.
If I can't find a favorite bookmark, it might be in a book on a stack next to my reading chair. Or upstairs by the bed. Or even on the shelf. Sometimes I barely start a book and put it back on stack without committing to it yet. That place might get marked even if I know that I'll likely go back to the beginning.
Before I turn a book back in to the library, I always check to make sure one of my bookmarks is not still tucked inside the book.