I want to know if this paper size exists in the countries where each of you live, ok? Can you easily find ordinary paper with those dimensions where you live? And do they sell even bigger papers in those same stores?
The largest kami I have seen for sale in the US is 35 by 35 cm. (about 14 by 14in. ) I got this at the convention. Large sheets like the ones you described can be purchased at art supply stores but they are not directly usable. Most are thick sheets, card and poster board of various sorts not foldable except for vary specific folds, but tissue sheets that can be backed are availabe too. Note the words "art supply". There are craft stores and other places that sell kami but not the bigger stuff. Beyond that you would have to either attach sheets together or make your own paper. Tom
