IMHO I would wait until your claim is denied before you go to the expense of private doctors. You really need a doctor who can fill out the form in a format the VA accepts. You need an in service event documented in your medical records, a statement by a doctor that the event more likely than not caused your disability and diagnosis of disability. Without these three things you will not get awarded. Research Caluza triangle on this sight for more in-depth information. There are doctors that know how to do this but they do not usually come cheap. Usually if you have a strong case, after a denial, you can get a lawyer to take your case and they can help with the nexus letter from a doctor.
I know that we should not have to have a lawyer to represent us but that is the sad reality of dealing with VA appeals. They get 20% of your award but 80% of an award is better than nothing.