In any case you will be looking at probably having at least 20 of each made to make it remotely cost effective. What row, key size and base color do you want to do them? Assuming you need the legends in those or close to those colors...
You could do them as dye sublimation with imsto (who is a member here with Chinese manufacturing contacts). Key base must be lighter color than legend, you will probably be more or less limited to white keys.
You could do them as doubleshots with Signature Plastics. Most designs should be doable, you would definitely need to rework the hand to simplify it a little bit.
You could do them as infilled keys with QWERkeys.
If you just need the legend, and the color isn't important you can do them as lasered one off with WASD keyboards.
Or... you could go with DIY ghetto approach and use waterslide decal with clear coating. Or you could print out inserts for relegendable keycaps.