The keyboard keys are manufactured into the unibody top case and Apple doesn't really sell a set of key caps, but the entire topcase. So the correct way to change things would be to take apart your entire mac and re-assemble it on the new frame. I don't know of anyone making replacement keys - just silicone overlays.
Put another way, when taking your mac apart, when everything is out and sitting on the workbench, you have the frame with all the keys sitting in your hands as a whole part.
Thanks to iFixit for the great photo and repair guides.
If you are near an Apple Retail store - I would certainly go in and have a chat with the technicians. Let them explain how many screws are there to do the work to replace all the key caps. See if they have any ideas how to help you out. Just know you will be asking them to rebuild your mac around some new plastic keys.
I don't know if the current DE keyboard aluminum is 100% identical to the US aluminum - but in the past the return keys and key spacings were enough different that you couldn't just pop the caps and have things work. The white plastic scissors underneath the keys were different due to internal cable routing and things not obvious on the outside of the mac.