HELP!!!! birth, death and marriage certificates

I was wondering if anyone could inform me how I go about obtaining these types of certificates in Norway. At the present I am interested in the marriage certificate of Erik Olsen and Gunda Pedersdatter, married 30 August 1873 in Christiania, Norway. This is the date of marriage supplied on both their death certificates in NZ, however I have been unable to find either of them in the 1875 census in Norway, and I know they emmigrated to NZ about 1879/1880. I have not been able to find any marriage for them in the LDS IGI. Erik was born 5 March 1850 at Glamsaas, Hedmark, and the only information I have on Gunda was that she was born 1849/1850 and her parents were Peder Hansen and Gurie. Noted on her death certificate. Can not find any evidence at all of a Gunda being born to a Peder Hansen. Gunda and Erik obviously had a son named Anton who emmigrated with them....he was only a baby born in Norway 1879. Once again I have no idea where. No record can I seem to find anywhere. Hopefully a marriage certificate will help me with tracing Gunda back a bit further. Thanks, Kymm HELP!!!!!