There are some national agencies and thousands of smaller ones across the United States. Many of these types of agencies are nonprofit groups. Some will not have facilities but will provide foster homes for animals in their area or raise funds for animal related projects. Other Tidbits Animal control and municipal agencies will have large numbers of animals moving through their facilities. Each year close to 20 million animals end up in shelters.
Animal control has to deal with unwanted animals, abuse cases, and stray animals of all kinds.
Adopting an animal from one of these facilities is a kind thing to do however, as a general rule, not much is known about the critter's habits or health conditions. When looking for a pet that is lost, you will usually have to go directly to the agency that handles the animal control services for the area. If you go into a humane society or private shelter to look for a lost animal, they might have referrals and contacts for you, but usually will not have your animal. Many people get confused and angry at humane society staff since they cannot provide assistance with this. Please understand that it is usually against the law for a private agency to take in strays.