OP, what do you consider the "true definition" of a nymphomaniac?
I believe a "true" one is someone who could be classified as a genuine sex addict; someone who, no matter how much sex they have, never feels a true satisfaction from it, which is why they keep doing it.
So, along that strain, I don't think you could even know which providers *truly* are, as I doubt they would divulge that kind of personal information to you or any client.
Now, if you mean a woman who just loves to fuck, and fuck, and fuck, that is a different story.