Electronic parts. There used to be several local shops and a Radio Shack where you could go in and just buy basic electronic components (capacitors, resistors, photocells, transistors, breadboards, potentiometers, you name it). They phased all of that out in favour of toys and trinkets before just folding.
Now I have to order on Mouser or Digikey and wait. Sure there’s WAY more variety, but I miss being able to just run & grab something I might have forgotten (or, let’s be honest, fried somehow).