This post of mine talks about the track ball not clicking. It also applies to the button on the keyboard. There are not any parts sold online to fix the physical buttons on your device. Read up about it and see if you want to try and fix it http://forums.pinstack.com/f257/blackberry_pearl_trackball_select_button_gone_crazy-109868/
You need to pull the device apart and find the M button on your logic board and clean it. What I mean by clean it is pull the tape back and clean the brass ring.
Do at your own Risk