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.