Once my wife needed to find some mineral oil for treating our wooden cutting boards, and went from one pharmacy (chemist) to another asking for it. Time and again she was told “no, we don't stock that” with the authoritative weight of someone who knew what it was and was also keenly aware of the totality of their stocked items. Finally, on a hunch, after being told once again that it wasn't stocked, she asked “Do you know what mineral oil is?” “Well, no. What's it used for?” “We need it for cutting boards, but people also put it into ears to soften wax, and it can be used as a laxative.” “Ah-- perhaps you could use liquid paraffin instead.” Out comes the pharmacy reference book, and there in teensy print under the liquid paraffin entry was the footnote “*Also called mineral oil”. We've had this happen time and again, so you should consider making sure a shopkeeper knows what you're really talking about before giving up.
This isn't an isolated incident; I've encountered a similar response when looking for certain woodworking equipment at a tool shop. OK, so this isn't enough data points to declare this a universal phenomenon here, but I'd advise creating thinking on the shopper's part when faced with the refrain of "we don't do that".