I like to go to the horses mouth so to speak. That means looking up a local box
wholesaler in the neighborhood. I have been using cartons that were designed for calender sales. I got them in several sizes and they will work with up to 11x14 prints in the right size. I mount the print, put in an overmat with a slipsheet and some bubble wrap to fill it up. Have sent prints around the world this way with no losses. There is actually a number for these containers. A box website may give you the info you need.
Best, Peter Schrager