Is it possible to replace the lock screen button?

After dropping the phone on a hard surface, the casing was damaged and the top of the phone came away. The lock button which was secured by the case fell out and I have been pressing the place where the button used to be to lock/unlock the phone. This no longer works and I am now looking for a way to fix it permanently. Do I need to send my phone off to be repaired or can I order the casing and repair the button myself?