There are indeed laws to protect homeless persons from discrimination and abuse; even some that provide support and dignity. Unfortunately, the effect of many local laws and practices is to exclude homeless persons and to criminalize homelessness. However, laws vary from state to state and locality to locality. You have picked a very broad topic, and I suggest that you narrow your focus to a state or county with which you are familiar.
Typically, inquiries may be made to local Human Rights commissions, Human Services commissions, law enforcement (You might want to ask for a "ride along" with a law enforcement officer, especially if the department has Community Liaison officers), or local charitable organizations focusing on services to homeless persons.
Take a look at our section on Assessing Community Needs and Resources, found under the Learn A Skill Graphic in the Table of Contents and related tool kit, for suggestions that may help you plan your study.