I used to have some rectangular sugar thingies from a Soviet airline from the 1980s. Not a help, really, but...
I guess I just find this topic really fascinating. I doubt that modern American food has much in common with military rations from WWII (although obviously, the sugar rectangle is still being manufactured), but if I come across something, I'll let you know.
[EDIT: NVM, you have contacted them] Domino sugar might still make that kind of sugar, but they might be the only ones in the US. It's not something that we use instead of cubes. We mostly use cubes (when using lump sugar) from what I have experienced. I doubt there would be enough demand for rectangular sugar to justify a store to pay for it to be shipped here.