Where did Nider get it. He says so, and the answer is the most instructive thing about the book. His witchcraft material came from two informants: Peter von Greyerz, a secular judge who had held trials in the Simme valley in the Bernese Oberland, and an inquisitor at Autun. Nider had not met a witch. He had met men who prosecuted people accused of being witches, and he wrote down what they told him.
Follow that chain back one more step and the pattern of the entire literature appears. The judge's knowledge came from what defendants said in his court, and what defendants said in his court was shaped by what he asked them and by the conditions under which he asked it. A confession made under threat records the questions, not the life. Nider then set that material down in Latin, in a respectable theological work, where it acquired the authority of print and of a learned author. Later compilers cited Nider. Later courts cited the compilers.
This is why historians insist that demonology is evidence about demonologists. Norman Cohn traced the same circular movement across the whole genre: the learned text supplies the shape of the crime, the interrogation elicits a confession in that shape, and the confession is then cited as independent confirmation of the text. The Formicarius is the clean early case, because Nider names his sources and they turn out to be two prosecutors.
It was first printed around 1475, some forty years after composition, and circulated widely thereafter.