I think if you are adamant about using bookmarks (as I am), at one time or another you’ve used anything you could find that wouldn’t mark or damage a book. I’ve used metal bookmarks, cloth bookmarks, crocheted bookmarks, free paper bookmarks from bookstores (I still have many of these, some from the late 80s), folded pieces of paper, envelopes, greeting cards, postcards, receipts, bills, napkins, pens. I guess the strangest thing I have seen used as a bookmark is a golf tee…really not that strange, when it comes right down to it. Readers do what they have to do!
One other thing: if you borrow a book from me, it will come with a bookmark already in place.