I think I found one
http://www.alllosangelestours.com/
according to their faq
Question: Are the vehicles used handicap accessible?
Answer:Handicapped vehicles are available to you for no extra charge. Just please let us know in advance and we can arrange the transport to you on any of our tours.
Viptours and charters. I know, because my mother used one to do the grand tour of L.A. When contacting them, you have to ask specifically for a bus that is handicapped accessible.Good luck.