Code of Mississippi Rules: Current edition may be held at Library of Congress. See that library's policies prior to visiting.
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.