Last Names

I have been looking thru the census for the yrs posted-and can someone tell me when the wife started taking her husband's last name? In my situation that is the only way I can find my great grandmother listed(even though I know her maiden name). They are all listed under Nicolai Jonassen in the 1900 census for Buskerud-either Drammen or Modum district. This makes it confusing as I cannot locate her in her fathers' house-before marriage. One my ?,where can one find access to birth records? Where would I write to? Thanks