On June 5, Montana holds partisan primaries. Technically all qualified parties in Montana nominate by primary, but the state has almost never before printed up any primary ballots for parties other than Republican or Democratic. Generally minor parties don’t have contested primaries, and Montana doesn’t print primary ballots for parties with no contests.
However, this year the Green Party has two candidates running against each other for U.S. Senate, so there will be Green Party primary ballots. See this story.