Stationery shops are my favorite and I honestly didn't expect to find such a good one in Prague,…read morebut I did! Papelote is a Czech brand and they mostly sell things from local / national illustrators and designers, which I think makes it even more fun. There's also more global things like washi tape and origami papers, though.
Worth a visit if you like paper, postcards, tape, etc.
And yes, it's indeed quite a bit more expensive than most shops in Prague, but cheap compared to similar shops in Western Europe. And hey, it's paper! There must be more paper lovers like me out there?! :)