We've probably all got our favourites... I normally go to 7zap for no other reason that it's the one I've become used to, though I'm sure the alternatives are just a useful.
Unfortunately I've also found it's not infallible and I'm guessing that's true of the others also.
Because of that I've got into the habit of double / triple checking before committing to anything shown... specially if relying on part numbers when spending my hard earned buying something.
I'm finding a few of the big name manufacturers (usefully ones who originally supplied VW) have their own specific eCatalogues, some are even aimed at the classic market / discontinued lines. It takes the uncertainty out of it, as you're getting the info. from the horses mouth as it were.
I've a few bookmarked and will stick up links on this thread, as and when I get the time.
They vary in format ... for example some are databases while others are PDF's of printed catalogues - you'll have to navigate them for yourself, depending on what your looking for.
Bosch first... (interesting to note it looks like they're in the process of setting up a 'classic' outlet, hopefully bits and pieces that are unobtainable now may be getting remanufactured?)
Bosch-Classic (https://www.bosch-classic.com/en/services/downloads/)
Ate seem to have already set up a 'Classic' arm, remaking discontinued components...
Ate (https://www.ate-brakes.com/catalogues/online-catalogues/)
Bosch (https://www.bosch-classic.com/en/services/downloads/)
Ate (https://www.ate-brakes.com/catalogues/online-catalogues/)
Hella (https://web.tecalliance.net/hella/en/parts/cars/assemblies?targetId=1654&typeNumber=1654#@brc/brands:Car;targetType:cars;mandator:hella/assem:VW%2520GOLF%2520Mk%2520II%2520(19E%252C%25201G1)%25201.8%2520GTI;targetType:cars;mandator:hella;targetId:1654;typeNumber:1654)