You can find bespoke assembly services with a lot of the vendors, it will usually run around 100$ extra so 300/400 for the keyboard which altogether isn't that bad compared to prebuilt ergos like the Ergodox.

These tend to be location specific but try to find regional resellers of the PCBs.

Also worth checking out r/mechmarket, i.e. https://redd.it/wvf9dz