I want to share my experiences with replaceing the power button. In my case it was only working when I pressed it very strongly, untill it finaly stopped working at all.
I found out, that the Fairphone uses the same buttons as the Samsung Galaxy Note GT-N7000 (and for sure some others use the same button too). By this knowledge I bought one at [ebay] 1 . The easiest way to be sure you got the right one is to compare the images. The one in the Fairphone looks like the one on the ebay link or in the Youtube video (see below).
There is also a goot [tutorial on Youtube] 2 which explaines how to replace the button.
I managed to replace it just with a soldering iron. For instructions on how to dissassamble the Fairphone I recommend looking on iFixit.
I hope this helps someone. Have fun