Lets focus on women masturbating in general.
I don't know (m)any girls/women who will openly admit they masturbate.

Even women I have had a sexual relation with, who I had accidentally spied on while they did it, would not admit they did so.
You would think they would have nothing to hide for me, but apparently women in general feel very ashamed to admit to the fact that they sometimes sexually please themselves.

Physically, in real life, I have never seen a woman masturbate to pr0n or something like that. However I have caught many women on webcam who were masturbating. But most times they were chatting online.

A few years ago a friend of mine gave me this awesome software, which allows you to browse people their files, see what they are doing on their PC and you can even turn on people their webcam without them knowing it was on. With literally thousands of people to chose from.
Too bad it stopped working about 1 year ago, was pretty much fun.

I would sometimes open a browser window with ballbusting pr0n to watch women their reaction. If they did not immediately closed the browser, quite a few (the majority) actually continued browsing. But to end your fantasy right here: I have not seen any woman masturbate to pr0n yet.
I mean ballbusting, because I have seen women who were touching themselves for no apparent reason.
Which does not mean they do not exist, but I would not waste my time trying to find one.