I believe its scattered all over the place, For example; 38 U.S. Code, Sec. 3034 (3)(d)(1) which states that GI Bill benefits can be used for flight training if three conditions are met:
-- That such training is necessary for the attainment of a recognized vocational objective in the field of aviation;
-- That the individual possesses a valid private pilot certificate and, on the day the individual begins a course of flight training, meet medical requirements for a commercial pilot certificate;
-- That the flight school courses meet Federal Aviation Administration standards and are approved by both the FAA and the state approving agency.
I dont know of one single source that you can point to for everything. I may be wrong though...