Depends where you live. Where I live we could not find it in my country some years ago. Mail order only. Now I can buy them if I travel 2,5 hours total one way so 5 hours total(most of it by train).

Here they do not know what a Brompton is if I bring it into most bikeshops. They think it is a broken childrens bike

This is the small wheels you find in the shop I used to buy mine from: https://www.sjscycles.co.uk/tyres/

St Sheldon is mentioning four different 16" sizes here: https://www.sheldonbrown.com/tire-sizing.html

Edit: And you do not buy bicycle tyres in a hardware store do you- unless you are stranded??