I got the lego parts from
https://www.lego.com/en-gb/pick-and-build/pick-a-brick but there are lots of third party sellers that you can find via
https://www.brickowl.com/ and
https://www.bricklink.com/I am currently waiting for some black 13 hole beams to replace the grey ones. (For some reason the grey ones ship fast from Poland but the black ones ship slow from Billund, so I got the grey ones when I was prototyping.) When I fit them I will be more careful to make sure that the injection moulding sprue marks on the sides of the beams are on the inside of the keyboard not the outside 🤪
I also have some acrylic on the way to go around the key block above the plate. I rather like the texture of black solder mask on copper, so I am not sure if I want to cover it up or if the acrylic will be too shiny.