I had the same problem. Fitbit told me there are no spares available but they offered a discount on a new Fitbit which offer I haven`t taken up.
If you have the broken parts you could try doing what I did which was to carefully replace the side button and superglue the small plastic clip (which had broken into two parts) to the main assembly. This has worked perfectly for me......until the display went dim and the display disappeared completely a few months later. But that is another story! (Fitbit still working and synching to laptop but no display so not a lot of use!)
