Any office-stationery store will sell archival boxes for A4/Letter size paper, as well as other sizes. These fit most negative sleeve pages quite well and are pretty much a standard product in wide use. I have three different (European) brands, sold as flatpack and assembled with stainless bend-and-clip fittings, which are within a millimetre of each other in size to fit standard archive racking systems. They really are everyday products, thanks to the need to keep paper documents.