Apple - Macbook white thingy below keyboard button is broken

They are sometimes called scissor clips. I have bought them from thebookyard.com, which is a Mac parts seller based in the UK. Apple is known to replace them for free as a walk up service at the Genius Bar and you of course can order official repair parts from a variety of sources.

Be aware that different keys on the same keyboard can have minute differences in the layout and size of these supporting clips even when the key cap size is identical.

Alright, it turned out that I'm looking for a so called B-Clip

The number of this piece is 221159039672 If you type in this number on eBay, you'll find the right white clip.

The other number, which didn't fit for my Macbook Pro is 221159039824

I hope this solves anyone's question in the future, as it was really a pain to find the right result.