I'm old enough to remember when supermarkets had boxes. All the goods they sell comes to them in big cardboard boxes, and and supermarkets would have a fenced off area where they dumped all those boxes, so whenever a customer needed a box to put their groceries in to take them home, they'd get a box from that fenced off area.
I haven't seen those in decades unfortunately. It was a great way to reuse those boxes.
I don't recall ever seeing one of those in the person-facing parts of the store, but I've not had issues either asking someone who works there in the store or going around the back of the store where they do loading and unloading and asking there.