There was another play, contemporary to Shakespeare’s King John, called The Troublesome Reign of John, King of England (also known as The Troublesome Reign of King John). Though we believe the play to have been fairly popular during Shakespeare’s time, we are not sure of the play’s authorship.
Most critics believe that Shakespeare took this reasonably popular play and worked his magic on it; there are some others, however, who believe that Shakespeare’s is the first, and the other is a knock-off.
Regardless, another possible source material is (no surprise) Holinshed’s Chronicles.