They're hard to find. I've gone to my local U-haul and bought a box meant to hold a set of golf clubs, and then cut the width of the box down to my size. When I buy new guns, I ALWAYS save the factory box so if I ever decide to sell on Gunbroker, I can put them back in there. I use another box (Uhaul box) on top of that one.
You have to make sure to pack the rifle well. Any shipper will deny your insurance claim if the firearm was poorly packaged.