Here's a free bookmark / ruler from Blue Sky. I had to disable my adblock program in my browser in order for the signup form to appear. It seems these are made for spiral binding planners, so I'm not sure if you can use or easily modify them for other notebooks.
This is not to be confused with the social media Twitter replacement Blue Sky, which you should totally join and follow HIF on! I'm also personally on there and check it much more than any other social media site these days.