You get it from the Directeur de l'etat civi (Director of vital records in Québec.
In your internet research vehicle, type:
directeur de l'etat civil.
No. 2 is the english version.
To get a birth record, you will need to know the exact birth or baptism date as well as the specific parish.
There is no province-wide birth or baptism registry and although the church records exist no one has ever made a repertory of baptisms for the parishes in Montréal to my knowledge; if was done it would be for few parishes.
With regard to a map of Montréal for the early 1900 I trust that someone else will come to your help.