This part has somehow fallen of the compressor and now A/C compressor isn't working.

Can anyone help me out where to get one of these?

 

they do do a repair kit but you need to look at see if the compressor is damaged, the splines, the bolt hole and make sure the compressor is not seized. I would recommend you look at replacing the complete unit.

 

My friend has an identical car, so we took this from his car and everything is working ok. I found it on german ebay: http://cgi.ebay.de/Klimakompressor-S...item439ea25211

They said that they can send me one for my car for 50€. Case closed

 

Make sure you check for oil around the outside of the shaft seal. If you see excess oil, you are losing refrigerant. You can buy a repair kit for that too, but specialty tools are required. Might as well replace the whole compressor then.