Vietnam and 50 year old shells? Maybe one of the former Yugoslav states? Trying to think who would have supplies like that of both sizes. Mozambique might be another candidate. Ethiopia? In any event, they are likely old, and I wouldn’t want to be near the sending end any more than the receiving end.
That’s pretty smart on your part, not shooting off Ethiopian 155mm in your back yard. But then again, you’re not a young Uke soldier about to be mauled by Russian artillery because Uke counter battery artillery is out of ammo, are you. Most of this NATO stuff will good old American manufactured and originally American bought or subsidized.