Enforcement Agencies
Responsibility for enforcement of laws dealing with service animals, and indeed access in general, falls upon several federal agencies, depending on the service or environment an individual is trying to access.  For example, the ADA has eight separate agencies with authority to enforce requirements of that law:

US Department of Justice
US Department of the Interior
US Department of Education
US Department of Agriculture
US Equal Employment Opportunity Commission
US Department of Health and Human Services
US Department of Housing and Urban Development
US Department of Transportation (Aviation Complaints)

Advocacy Organizations
While people with disabilities can have the right to bring suit when there is a violation of accessibility law (as under the ADA for example) it can often be very time consuming and costly.  There are a number of disability rights organizations that are charged with promoting voluntary compliance with accessibility laws and provide technical assistance without engaging in litigation. 

Centers for Independent Living

• Centers for Independent Living (CILs) provide a variety of services that allow individuals with disabilities to live independently in the community. At least 51% of the staff at any CIL must be made up of people with disabilities. CILs can be found in every state and territory.  You can find an online state-by-state listing of CILs at http://www.ilru.org/projects/cil-net/cil-center-and-association-directory.

The ADA National Network

• The National Institute on Disability, Independent Living and Rehabilitation Research within the US Department of Health and Human Services provides funding to 10 Regional ADA Centers in the United States.  These Centers provide technical assistance and guidance to promote voluntary compliance with the Americans with Disabilities Act and are often well-versed in other accessibility laws.  Each Regional Center covers a handful of states (including the US Territories) and provides technical assistance and occasional small amounts of funding to local advocacy groups (such as CILs) who are trying to make their communities more accessible and inclusive.  You can learn more about the Networkand find your Regional ADA Center at http://adata.org/find-your-region.