A long time ago we found this cartoon from Charles Barsotti, except there were two obvious differences.
One, it said "Superstore" instead of "Sainsbury's" because on this side of the pond most folks don't know what a Sainsbury's is. And apparently it appeared in the UK mag Punch first, from what we can tell.
You can see the American-friendly version here. I believe this is the company that produced the card we had found. Update: Alas, the stateside card seller is gone, but you can still get a print of it from Punch.
Two, it was obviously lacking the bigass copyright symbol.
Regardless, this is, bar none, my favorite non-Far Side single panel comic ever. I make that distinction because Far Side is sort of, you know, this massive sprawling entity all its own, no?