I think it'd be cool to read up how these people who believed in this so strongly supported it. I'm not sure if this is really the right place to ask for things like primary sources more than explanations, but I Google isn't being all too helpful for for this.
Some of the strongest defenses of divine right come, unsurprisingly, from the period of it's decline. King James I of England wrote two works on monarchy that might be of interest, "The True Law of Free Monarchies" and Basilikon Doron" in 1598 and 1599, respectively.
If you want something from the medieval era, I might suggest John of Salisbury's Polycraticus (1159).