Some caregivers have the mistaken belief that the disabled person's teeth are not valuable to them and so care is neglected. On the part of the disabled person, there is sometimes an inability to understand the consequences of neglecting dental hygiene and care, or there is an inability to cooperate with dental treatment. Many disabilities interfere with the ability of the person to perform the fine motor skills needed to clean teeth and gums. This will probably be a general dentist or pediatric dentist. Don't be afraid to take your grown adult disabled children to a pediatric dentist.
Some pediatric dentists treat disabled adults of any age.
If your city has a dental school, call the department of Pediatric Dentistry, Community Dentistry or any Advanced Education Programs for general dentists. These are called AGD or GPR programs. I'd suggest not going with a regular dental student but seeking out the faculty member that teaches the students how to work on disabled patients. Ask for advice from some of the parents you meet who have older children with the same disability as your child. See how long they have been going.