From a local office supply store. As long they are in they A7 industry standard they will fit. I also found them on Amazon US and Amazon UK, but for some reason they are much more expensive there. I paid about 3€ for those.

Amazon US and Amazon UK, but I would strongly suggest just checking out a local office supply store for any index cards that meet the A7 industry standard. They will probably be much less expensive there and it supports a local business.