Hi! Where can I find the Arduino Mega bootloader?
It doesn't appear to have been released yet. The Arduino boot-loaders are all based on the same code-base and it is certainly not an Arduino invention.
You will also need the board definition file for the Arduino-Mega as well before you can properly use it with the Arduino IDE.
I should have checked first. The Mega bootloader is in the standard Arduino-0015 distribution.