For best results from play testing recruit according to the demographics of your target audience. For average gamer age, aim for around 20% of playtesters to represent a mix of the 4 main types of impairment (motor/visual/cognitive/hearing), or slightly more if your game involves speech chat/recognition. For children, focus on motor and cognitive impairment. For over 65s, ensure all groups are represented, and increase the proportion to 50%.
If you work for a large studio / publisher and use a recruitment agency to find participants, simply adapt your recruitment profile.
If you don’t have this available, both social media and local community groups can be good recruitment grounds. Universities/colleges also often have a central contact point for students with disabilities.
Best practice example: Something Special: Out and About (video)