Administrative Rules of Montana: Current edition may be held at State Law Library of Montana. Georgetown Law Student may request scans via Interlibrary Loan.
In Westlaw, see also historical regulations, currently proposed & adopted regulations, and regulation tracking which can be found on the state's Regulations page.
State registers and regulation tracking are offered by Lexis.
In Bloomberg Law, proposed, final, emergency, temporary, and interim rules, plus regulatory notices can be found under Rulemaking within a state's Regulatory and Administrative page.