I'am trying to find a website for complete rosters for every football
team in the history of the sport.Where would I go?

Bravesfan --

I don't think that you'll find it.  There is such a website for
baseball, whose fans have long been stat hounds (thus we even have
SABR, the Society for American Baseball Research).

However, football sites are incomplete, such as this one:
http://www.pro-football-reference.com/

I can tell you where in print you can find early rosters to help fill
out the data.  Some of these publications are online, but are
difficult to search because of the structure of the database.

Best regards,

Omnivorous-GA