How do I get a copy of a cell phone bill other than my own. Is there a web site ?

You don't.

Unless you are on the phone plan, that's an invasion of privacy.

Let me guess: You suspect your significant other is cheating, and you want a copy of their bill so that you can find out for sure by looking at the numbers on the bill.

If that's the case... why not just ask to see a copy of it? If they have nothing to hide, that shouldn't be a problem, right?