I'm sure you've run into this one - those two-lightbulb table lamps (and probably also ceiling fixtures) where the conductors enter these metal-shell lampholders from the side.
Where do you get replacements for THAT???
Another thing are those cardboard sleeves and insulating disks.
I wonder if there's any aftermarket company that sells those - the lighting parts stores in my area certainly don't.
Sometimes when you rewire a table lamp (and I've rewired a bunch of them), the switch and/or the shell are still good and it would be cheaper and easier to replace just the rotten paper insulator (and maybe a new interior mechanism) than to unscrew said socket shell and then fumble with nuts, nipples, lock-washers etc. until the lamp is properly reassembled again.
Sometimes it's easier to replace the metal shell lamp holders with bakelite ones...no paper sleeve to worry about.