Google "NickNames Database" and you will find a number of sources, for common English names. But there is no universal list of ALL first names. In my lifetime, I've seen such unique names as Moon Unit and Will.I.Am.

By the way, William is the proper first name. Bill, Billy, Will, and Willy are nicknames for William. This isn't to be confused with the female name "Billie"... or with people who were named simply Bill by their parents. 🙂

Having looked at this problem, it seems that we can't just arbitrarily settle on a single name. We have to generate all combinations to search on.

Good luck.