With a little help there's no reason why you can't train your own service dog. The advantages are many - not least of which is the cost.
I need some specific traits in my service dog, so I deliberately looked for a puppy who would have some stature as an adult.
My choice of breeds is not one that others would find a good selection, but Mastiff x Rottweiler x Bouvier des Flandres is what I've gone with. Each of these breeds on their own is trainable, and used for protection, support and farm work. So my thinking is that this type of mix will be reliable and calm (eventually).
So it begins. I've found some good resources, including one forum and website which teaches clicker or marker training, and a Facebook group and affiliated website solely dedicated to training a personal service dog.