I think the most familiar to keyboard hobbyists are JLCPCB, PCBWay, and OSH Park. I’ve had success with JLCPCB but cannot speak to their assembly service.
If you want a PCB on the cheap and with good enough quality, then go with JLCPCB: https://jlcpcb.com/
Beware, it does very limited PCBA service so it is fine only if you can solder your components yourself.
Another one is PCBWay: https://www.pcbway.com
This one also fully does PCBA service (never tested) and quality is looking to be better than JLCPCB but it is also a bit more expensive.
This company has been used by Gondolindrim for some of these PCB designs, including the PCB of the Austin keyboard.
I’ll also add Elecrow to the list. The one time I had PCBA done, I used them and was quite happy with the quality. I’m actually considering doing another PCBA run, and was thinking about using them again.
I found someone on the MechKeys Discord in the #kb-design channel. Haven’t received the design files yet but so far everything seems to be working out.
I’d love to say Makerfabs. I had a pretty good experience with their pcb assmbly service, friendly to makers / hobbyists. Also, I am satisfied with the CE certification service they did for me.