Source material is abundant but an open mystery because there is no uniform agreement on the authors.
The book claims celestial authorship but at the same time claims the best human sources and concepts were used whenever possible; over 1000 human contributions were used (or some quote to that affect).
Some scholars researching human sources are found at Square Circles Publishing.
A friend of mine is woking on compiling comparisons of major religious texts and the Urantia Book for questions such as yours (below), but I'm afraid I cannot adequately answer your source question myself (which is why I hang out here at GH.com, because it is filling in those gaps).
The Urantia Book and Major Relgious Texts (forum)