This sounds very similar to what you're looking for: https://thenoteboard.com/ https://thenoteboard.com/. Sadly it doesn't seem to be available right now, but i

This sounds very similar to what you're looking for: https://thenoteboard.com/ https://thenoteboard.com/. Sadly it doesn't seem to be available right now, but it's a pocket whiteboard made up of 3x5 segments. It folds down rather than being a stack of cards, but it may work for what you're thinking of.