I have no experience with this, but what Hartzell said makes complete sense in this culture.
In fact, I think you could gather your own evidence just as well, if you really wanted to. You obviously already know there is someone, you seem to know who it is, and I can't imagine it would be very hard for you to prove it on your own.
But the fact is that if you want to save your marriage it simply doesn't matter who he is, or if there's proof. Your wife wouldn't be with someone else if there weren't a reason. Either this is someone she was in a relationship with before and not really gotten over, or she's met him because there is something lacking in your marriage, or some problem that she hasn't been able to work out with you, or something similar.
If you go about trying to get evidence of your wife's affair, you will only cause a greater divide between you, as she will very likely resent her privacy being invaded, resent being made vulnerable legally, resent that this other person whom she may care for is also now vulnerable legally, and you will be the cause of all this resentment. How are you supposed to expect her to re-build a marriage with you then?
I know that SHE is the one who is cheating, but if you will still have anything left with her, then you will have to forgive her. Having photo/video evidence of her transgressions would seem to me to make that a lot more difficult.