The original indoor incandescent light fixtures (the white dome lights used by TM) are very common items. Start with a Search for "dome light" at Camping World, and if that doesn't work, go to one of the big RV supply places like pplmotorhomes. For example, is this the one you are looking for?
http://www.pplmotorhomes.com/parts/r...dome-light.htm
Bill