Was hoping to see more discussion here. When I maintain them, which I don’t do enough, I tend to go to a site depending on the make.
Mercedes seems to have great part service in house, for Citroen and Porsche I use an aftermarket reseller (online), for the MX5 NA there are lots of online options (even Ebay).
I have sourced second-hand parts but it takes a long time.
Common things, like batteries or generic tools, I source in local shops. It’s globally produced but they can give good advice on battery chargers and the likes, plus we all know we should buy locally when we can.
I used to order motorcycle parts from Great Brittain, but with Brexit I’ve completely stopped that. I have not found good alternatives there.
I can only really speak for Mercedes as I own two W124s, but as you said, their classic parts service is very good. Though I feel like it’s gotten a little worse in recent years. Luckily there’s also a huge amount of 3rd party manufacturers. Very specific stuff like wiring harnesses, interior panels and such are best found used on eBay and similar sites. Having the workshop documentation and parts numbers is a must for older cars like this.
Same here. VW Heritage parts has a decent selection for the cars I work on. And for T3 (Vanagon here in the US), there are a bunch of domestic parts suppliers, same for air cooled Porsches and VWs (Beetles, Buses, Things, Dune Buggies).
I’m also rebuilding a 64 Chevy truck for a client, and almost every body panel is available from Taiwan, though so far Auto Metal Direct (AMD) has had the best out of the box fit. Some cheaper panels look like they were designed by a blind man wearing baseball mitts.
