"Some Class B dealers have also been known to obtain animals from unregulated middlemen known as 'bunchers,' who have been documented acquiring lost, stray and 'free to a good home' pets," the HSUS factsheet on Pets Used in Experiments reports.
Luckily, there have been some recent wins in this area. In 2015, the largest Class B dealer went out of business and the USDA is prohibited from licensing dealers in the coming year, according to HSUS.
"Dogs and cats occupy a particularly important place in American society in their roles as companion, work, and hobby animals," a 2009 government report observed:
In addition, they serve as important animal models for research that has advanced both human and animal health. This multifaceted relationship with humans has fostered an uneasy tension between general society and the scientific community, and this tension has intensified as the stature of pet dogs and cats has risen in many households to that of family member. It is thus not possible to accurately assess the desirability and necessity of using random source dogs and cats, and in particular those from Class B dealers, for research without taking into account public perceptions, the impact of the animal protection movement both on public attitudes and on the availability of these animals for research, changing trends in the use of animal models for research, and responses of the scientific community to all of these factors.
Some groups, who know how nuanced and complicated the question of animals used in medical research is, are working to find alternatives in the long run to reduce the use of animals in labs.