Haven't had much of a problem, really.
That said, OEM CORRECT stuff is harder to find than very-similar aftermarket stuff.
And old-school rebuilders are aging out/retiring (radiator shops, for example).
Cap & rotor I usually use the tan Accel stuff.
Bearings & seals are numbered and can be found that way (Timken, etc).
Some stuff, I tend to update to more modern internal operation (voltage reg, distributor internals, etc) in the OEM housing/cover.
That said, my old cars are usually pretty bare-bones: no PS, PB, PW, AC.
I am just about out of my stash of correct wide-flute headlamps, though.