Where can I find someone who can help design a bathroom for my disabled father?

That should be a simple thing to do if their is room on either side of the door to the bathroom. When I was using a wheelchair and then a rollator after surgery on both knees, I couldn't walk into the bathroom on my own so we have a handyman (you can get someone who does carpentry or get a contractor) come in and he took off the door facings, cut several inches off the sheetrock and replaced with new door facings, painted and all looked like it was built that way. The doorway was now about 5-6 inches wider than before and the wheelchair just barely make it thru but it worked. Contact a reputable contractor and ask if they do the work you want. Have him come out and explain in detail what you need and see if he could do it. Pick one who advertises bathroom renovations and then see what he can do.

When I needed some rails put up around the house for my dad I contacted the Americans With Disabilities. They came out and did a great job. I'm not sure if they can do an entire room but it wouldn't hurt to ask.

I would first contact the Westchester County Area Agency on Aging for recommendations. There is a family owned outfit called Walsh that I've seen advertisements for in local Westchester newspapers and magazines; I have no personal knowledge of them.

I would start by going to a medical supply store. Usually the room as a whole is too small, doorways too narrow. A typical 5x8 foot bathroom is unworkable.