Andy, I know I've seen large book like binders that daybills and Italian locadinas fit into nicely but I have no idea where you could purchase one. Possibly at a art supply store?
I know of one collector who stores his inserts in custom sleeves he makes himself. He takes a piece of foamcore, NOT brown corrugated cardboard, and cuts it down to a larger than insert or daybill size. He tapes a large piece of plastic or polyester to the back around 3 sides. In this way the plastic is secure, it's top mount, and you can store as many as 5 to 8 inserts in one sleeve, depending on how large the foamcore is cut. Foamcore used in this way is absolutely safe, it's harder to bend than corrugated, and the inserts or daybills are fully protected. Brown corrugated cardboard contains acids that will soak into the posters and in time will brown the surface of anything coming into contact with it.