I had never heard of Witkin before so I did some looking.
It is interesting and different. As usual, some of it I liked, some I didn't. All of it was original.
But, back to your question on how he got his subjects. I have only one word:
OK. Maybe a few more. If you don't know, Michealangelo did something completely illegal to learn about the human body so he could paint and sculpt. He snuck in and disected them. da Vinci did similar things.
Yet both of these men are revered today as masters of art.
There are always going to be those who work outside the box. Witkin appears to be one of them. What he does with his art is what will eventually determine how he is judged. Many living at the time of Michealangelo were scandalized by his nudes. Today, they are considered masterpieces but, over the years, many have had clothes attached or painted onto them.
I would need to see an artists body of work in think on it to decide what it is to me. We need to remember, however, that great artists are often not entirely in step with their times.