These were kind of surprise finds.

Section 443 (Domestic raising of fur-bearing animals; classification) of Chapter 17 - Miscellaneous Matters of Title 7 (Agriculture) of United States Code.  Hosted by The Legal Information Institute at The Cornell Law School.

Rules and Regulations under Fur Products Labeling Act contains some examples using muskrat.  From Arent Fox Advertising Law Internet Site.

Captive Wildlife Regulations gives "Standard Caging Requirements for Captive Wildlife", including muskrat.  From Florida Fish and Wildlife Conservation Commission.

Territorial Lands Act, Chapter T-7 covers prohibition against sale of and leasing restrictions of territorial lands suitable for muskrat farming; from Consolidated Statutes, Department of Justice Canada/Ministère de la Justice (Canada).

Article 41:08 - Furbearer Seasons and Methods from Administrative Rules, Legislative Research Council, State of South Dakota.

Muskrat (rata almizclera in Spanish) are mentioned in Costa Rican custom tariffs dealing with animal skins (Peletería y confecciones de peletería artificial o facticia) and animal hair (Lana y pelo fino u ordinario; hilados y tejidos de crin) from Negocios - Arancel de Aduanas - Costa Rica - Infoweb.

Muskrat (again, rata almizclera in Spanish) is mentioned on Tratado de Montevideo - Resolución 78 del Comité de Representantes - Anexo 1 (Treaty of Montevideo - Resolution 78 of the Committee of Representatives - Appendix 1) and México-Costa Rica / Lista de Desgravación (another page in Spanish) lists muskrat among the items in the free trade agreement between Costa Rica and Mexico.  Both hosted on SICE - Sistema de Información sobre Comercio Exterior (Foreign Trade Information System).

Arancel de Aduanas (also in Spanish; Capitulo 51) uses muskrat as an example of "fine fur", to differentiate it from wool or "ordinary fur".  From TLC Mexico - Costa Rica, Costa Rica - Viajes - Turismo - Negocios - Infoweb S.A.

There is also a page (again, in Spanish) dealing with muskrat and Chapter 43 from Mexico:  Tratado de Libre Comercio México - Costa Rica (Free Trade Agreement Mexico - Costa Rica) hosted on Subsecretaría de Negociaciones Comerciales Internacionales (Undersecretary's Office of International Commercial Negotiations), Mexico.

Here's a couple more pages (in Spanish) dealing with Chapters 43 and 51:  es43 and es51.  These are hosted on CONEX - Douane & Informatique (a site owned by ORNIS in France).

Links to rules and regulations about hunting and trapping muskrat (usually in a fairly readable form) appear on my Trapping/Hunting page.

State Wildlife Laws Handbook allows you to view each state's wildlife policy, protected species of wildlife, and habitat protection, although the information is getting old.  It was produced in 1993.  From Center for Wildlife Law, Institute of Public Law, University of New Mexico.