Data of the french magnetic observatories have been published in "Annales de l'Institut de Physique du Globe de Paris" until 1964.
Observations carried out between 1965 and 1986 have been published in the "Observations magnétiques" yearbooks of IPGP, EOPG Strasbourg and ORSTOM.
Since 1987, data are published by BCMT in its «Observations Magnétiques» yearbooks. These publications can be downloaded in pdf format here and can also be obtained by writing to BCMT at the following address:
BUREAU CENTRAL DE MAGNÉTISME TERRESTRE
INSTITUT DE PHYSIQUE DU GLOBE DE PARIS
1 rue Jussieu
75238 PARIS Cedex 05 - FRANCE
The historical publications, recently digitised, are available online (pdf format) here.