Midori calls it Cordoba paper. The closest thing I can compare it to is the paper that jeans labels are made of—tough and difficult to tear/rip.
Muji did a range of notebook covers called Denim Tag. They were made of the same material they make the tag on the back of your jeans with. They had A5 and B5. Can't find them now but keep an eye out should they reappear. Both sizes were under $10 and I use them everyday. Amazing value and quality - hope they reappear at Muji soon as I would stock up immediately.