I, an American, am now having trouble using the word “store” to designate the retail establishments in which you buy stuff; those are called “shops” in British English. Here, “store” means “storeroom” or “storage room”, though I doubt the short form “store” is used in the UK…
“Stores” can also mean “supplies” or “inventory”, but the word you’ll hear in shops here is “stock(s)”. If a shop has run out of a particular item, the shopkeeper will say something like “no stock” or “got no more stock already” or “stock finish already” and probably also make a waffling motion with one or both hands.
Interestingly, you can write either:
While stocks last!
While stock lasts!
but my guess is that the second one is far less common in part because the “sts” consonant cluster at the end is hard to say. I think it also makes sense to use the plural version of “stocks” because typically, the shop is selling individual items, not something measured in volume or by weight, so using the mass noun would be a bit strange.
“While stock last!” is just wrong, but that doesn’t mean nobody writes it.
So anyway, today I chuckled when I saw a sign on a door near a public restroom that said “janitor store”. Surely it’s not a place to buy janitors, though with a bit of imagination it could be a place where janitors shop…