Outside of a municipality or subdivision you will be responsible for providing water. The most common sources of water are groundwater wells.
Do not assume that groundwater will always be easily available. Before buying a property that lacks a well, you will need to determine the groundwater availability at the site. Talk to the neighbors, local well drillers, and the county planning board. They have intimate knowledge of the local groundwater conditions. The Montana Bureau of Mines and Geology has well logs for local groundwater developments and is a wealth of expertise and advice on all subjects relating to wells and groundwater. Once the well is developed, you will need to file a Notice of Completion with the Montana Department of Natural Resources and Conservation to obtain a water right for the well.
Water well drilling for the prospective owner provides detailed information on drilling a new well.
Montana follows the Prior Appropriation/Western Doctrine for water rights, where the right to use water is determined by historic use and availability. Water rights are established by putting water to beneficial use, and the earliest water rights have priority in times of shortage. Water rights may or may not be attached to the property when you buy it, so check with the local Montana DNRC Regional Office to see if you can use the water and how much. There are many examples of homeowners along Montana rivers illegally putting small pumps in the river to water their lawns without water rights. When in doubt, do not use river water until you are sure that you can legally do so.