I may have mentioned I'm slowly refurbishing the bike I built when I was 18. It's a bespoke-made touring bike made for me, so while it's cost a hideous amount of money, it is one of my favourite things.
One of the difficulties was the brake lever "hoods" - rubber items that go over the brake levers. They were pushing 50 years old & shattered when I took them off. Replacements are available, but out of stock, but I managed to find a pair BUT, the cable adjusters that poke through the rubber hoods are supposed to have rubber covers. mine were similarly brittle & strangely - in spite of being a much simpler shape, are not available at all.
However, I've recently been experimenting with flexible 3D print filament & the OnShape on-line CAD system & the upshot is.......Pretty good I think