like these?

https://www.rejuvenation.com/products/bowman-toilet-supply-line/

https://www.rejuvenation.com/products/waterhouse-toilet-supply-kit/

https://www.build.com/product/summary/1887482

https://www.build.com/product/summary/1887480

https://www.build.com/product/summary/1879562?uid=4461379

(some appear to be "special order", i.e. long shipping time...)

Here is a Kingston brass that might work. Says it is brass

cover for toilet turnoff

You can buy just the cover too

cover alone

I would steer clear of Kingston Brass anything as the entire brand is known to be of dubious quality.

Corrosion happens between two metals. If you have pex, a decent valve won't corrode. All valves wear out over time. A saying in the plumbing trade is that old wood develops patina, old plumbing develops leaks.

as well as work when you finally do turn the thing?

Ah, the 'when you finally do turn the thing' is the problem. You should operate the valve every so often to keep it in working condition. This helps to keep mineral deposits in the water from causing the valve to seize up. And make sure you get quarter turn valves so you don't mind the maintenance.