Hello David and a very warm welcome to EOS
If you know exactly what you want I would think a pharmacy would be your first option. They may not stock what you want but they would be able to order it for next day, that’s if they can be bothered.
It is possible that the product you want may have been discontinued. However I doubt it was anything unique made at a specialist manufacturing plant? Usually it is a few manufacturers that will fill anyone’s bottle. It’s a bit like the Helen Mirren advert for L'Oreal Paris, because your worth it, made in Taiwan from the same batch as Poundstretchers own.
Good luck and let us know how you get on.