There are vendors that make reproduction boxes for Lionel postwar.
For anyone else, I personally don't know (beyond specialty cases for transporting multiple cars or locomotives).
For other brands, "good luck with that". A number of Forum members make no secret that they discard rolling-stock boxes. Perhaps we should set up a clearinghouse to mate unwanted boxes with folks looking for rolling-stock packaging. (I could use boxes for an MTH 50' boxcar and RailKing SP-style caboose. I could also use a trio of Menard's boxcar cartons with the clamshell insert. I can compose and print my own labels for 'em)
For more generic rolling stock, I made a template from an MPC box to construct my own MPC-style boxes for cars up to 12" in length, complete with transparent window. Photos are in this thread from June 2015, which also discusses some other options.
---PCJ
(2021 edit: still looking for the above. Add a pair of Menards' three-dome tank car boxes with clamshells on top of them)
