Mont. Code Ann. § 42-1-104
Venue
42-1-104 . Venue. (1) Proceedings for adoption must be brought in the district court of the county where the petitioners reside.
(2) Petitions for appointment of a confidential intermediary may be filed:
(a) in the county where the decree of adoption was issued;
(b) in the county of residence of the petitioner; or
(c) if the petitioner resides out of state, in any county.