Hi,
If the navigator buttons are not literally broken but just doesn't work, it may be easier to fix the navigator buttons than to find a replacement.
Here is a link that shows you how to fix a remote control unit, but the principle is the same for your phone keypad. In the video it shows using a small screwdriver to pry open the case. It may be easier and cause less cosmetic damage if you use a wider bladed tool such as a 3/4" paint scraper. It spreads the pressure over a wider area.
https://www.youtube.com/watch?v=gVr8eLW0-_o
Hopefully this is of some help.