They're widely available online or even in person at auto parts stores, which in rural places double as tractor parts store. My local NAPA is quite good at tractor parts. This seems almost a miracle to us but it's possible specifically because old tractors were designed for repair. They have shockingly few parts when compared to modern ones, and the parts that they do have are so, so much more common across different makes and models.

I usually have an easier time finding parts for my 1950s tractor than my fucking 2007 GMC Canyon, which also breaks more often than my 70+ year old tractor.