Specific advice on coin mechanisms and locks is on those pages. Those are the only parts that are generally interchangeable between all doors.
If you have a modern SUZOHAPP door, great! You can go to their web site and order parts. But you’re better off going through a reseller, as Happ’s markup is substantial. Third-party sellers seem to have the same parts for less.
Happ probably still has some parts for their older WG/Coin Controls coin doors, too, but it looks like fewer and fewer.
For manufacturer’s doors, check the pinball suppliers. Pinball Resource has reproduced many of the inserts. Marco Specialties and Pinball Life have a limited selection of parts.
For Coinco, good luck. You may need a donor. I’ve had some luck with Arcade Boneyard. I have seen some NOS parts for sale from Pinball Resource in the past on their Specials page.
I’ve found some Entropy stuff from Pinball Life in the past, notably reject buttons.