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Not going to lie, I've wondered this for a while too.
I got a box when I bought my skis that I held onto incase the day came that I needed to send them back or sell em but I imagine you could take your skis to the post office and they would be able to help you out.
If you have a ski shop in your town go there and ask, they give you them for free. If not, or if you are shipping skis with bindings, then go to a local furniture store, ask for some boxes that like a desk came in or something. They're usually huge so you just make a box out of it. I've shipped out some pretty ghetto looking boxes in my day. Just don't use duct tape.