Small shoe sizes.
My wife has really small feet - size two or three. Shops never have her size.
The assistants always say the same thing. "We only get a few small sizes in and they always sell out really quickly."
To which I always reply "well why don't you get more in as there is obviously a demand?"
They always look at me as if I'm from mars. Or scowl at me as if I'm the most arrogant arsehole on the planet for suggesting such a thing.