You can get married in Justice Court by a Justice of the Peace. However, walk-ins are not available and you must make an appointment by calling 406-256-2771 or emailing jccivilclerks@yellowstonecountymt.gov. The cost is as follows:

Weekday here at courthouse or another site ($80)
Saturdays ($100) (if a Judge is available); and
Sundays or holidays ($125) (If a Judge is available); and
Round-trip mileage if the wedding is out of town. All funds are cash only.

What Justice Court needs to perform your wedding?
Marriage license and fees
The marriage license is obtained through District Court 406-256-2851 or https://www.yellowstonecountymt.gov/clerk_court/marriage.asp