Note to male readers: this is harder than you think. The first time I tried (my first wife was working, I was not, so I did the grocery runs) I saw that ah, she was almost out, and added them to the grocery list, patting myself on the back for being a thoughtful partner. And thus set myself up for my first confrontation with the multi-choice hell that is the "feminine hygiene" section. In a modern world I would have just called Linda to ask, but this was 1990, so I just picked a package that looked familiar and brought it home. It was wrong, of course. With a background brief and careful notes I returned to the grocery store the next day and picked the two types Linda preferred (also my introduction to One Type Does Not Fit All Times Of The Month).

Current wife was a tampon + pad user until menopause made it (mostly) irrelevant. Again, they had to be a particular brand and type.

It's not easy out there, guys! Cut your partner a break and pick up the damn menstrual supplies!