I accidentaly broke the plastic joints between the 4 shoulder buttons because I didn’t have scissors or pliers at hand. Could I get a replace for just the buttons or do I have to buy the whole kit again (which of course I will not do)?
Why does not Clockwork sell separate parts for at least the more weak parts: cables, plastic buttons and covers? It’s very frustrating if something goes wrong when assembling the unit.
I agree, it would be nice to buy some spare parts if something goes wrong.
Anyway, I’d suggest to look around for someone with a 3D Printer in your area. Universities are always a good way to start looking at.
If you damaged some cables/connectors it shouldn’t be a problem to get some replacements in an electronic store nearby, if you bring them.
I’m not sure if the open source policy covers these small components.
I’d have a look at the specification in the docs and look for a matching product in the web.
If you live in Germany, I’d offer to print some parts for you (free), as long as my device is capable of it and you pay for the shipping.
One thing I must say though…damn I really like this community! Everybody is very helpful and friendly. That is a BIG plus. 
The lightkey components are given away as STL files (3D printable files) on the website’s store page but I can just drop the relevant files here for you.
Here you go!
https://mega.nz/#!FdEVHARQ!81qFs1oogxFed9RmqMPhhSgP84LSSyHbauF8Y9FZRKE
The part I think you described in your post should be GameShell_LK3.stl.
Thank you too much! 