To get to the interior bulb, you have to remove the wooden panel on the front. That allows you to remove the front grill on m/c. Standing on a stool, you can then reach in & pull it out. Bulb is halogen, so use a paper towel when installing new one.
We ordered ours online at GEAPPLIANCES.
http://www.geappliances.com/service_and_support/
Part # has changed since we got ours. If it's still under warranty, this is also how we got service to get the first bulb replaced.