Where I live in Spain the off-the-shelf variety is limited and of poor quality, so it is unfortunately necessary to go into a pharmacy and ask the pharmacist.
My Spanish is pretty limited, but I learnt the words 'adultos panales para mi abuelo por favour' in preparation. The pharmacist bought out a line of pullups, and I realised I should have also worked out a way to communicate that I wanted slips.
Eventually she pulled out a big back of some fabric feel slips by a company called Lysette (they sag awfully when wet but have a very authentic look and feel). I said yes, keen to conclude the transaction as quickly as possible, and threw her the money.
As I was about to leave with the pack, she called back holding a big box. It transpired that they only sold these things in bulk, and I had unknowingly brought a big box full of 4 packs (80 diapers in total), which was far more than I could carry or hide. Plus the cardboard box had a big picture of a diaper on the front, so wasn't even close to inconspicuous.
I tried to wave her off, but I couldn't communicate this to her. At this point a lengthy queue had developed, consisting of people all curious to know why a twenty-something foreigner with no Spanish was trying to buy medical grade adult diapers in bulk.
Eventually I had to relent and grab the huge box off her. I waddled down the road as fast as I could to dump the excess in a dumpster, hoping against hope I hadn't been spotted.