Don't know whether the Interior light is the same, but from what you are describing it may be. I've had to replace the bulb over the cooktop (the under cabinet fixture) of the GE Profile Microwave/Convection we have and I got it at HD:
It is a Specialty #16809 GE edison 25 halogen - came in a two pack - 25 watt, 120 volt, G8 base, T4 bulb
States on the package "Replacement lamps for GE's undercabinet fixture".
May not be the same but FWIW maybe you will need the info another time.
On edit: I just took a mirror and could see the base of the bulb inside the
microwave - I could read "120V 2something" - could not make out the second number in the wattage.
Just found this link to a bulb site:
http://www.buylighting.com/GY8-120-V...ipin-s/187.htm
Very top bulb on the page may be the bulb.