How can I obtain original box/packaging for 2019 Mac Pro?

No one from Apple participates here. We're all just fellow users. But there's no way they will have the original packaging for any 2019 Mac. I doubt they'd even have the original packaging for a current Mac, not that they could send out. All their products are built and packaged overseas and then shipped here; there's not a stock of boxes sitting in California somewhere. Your only possibility of finding an original box would probably be if you have a Mac users group in your area and someone would just happen to still have the old box lying around. Or you could post a notice in some place like Craigslist and see if someone can help.

Regards.