How do I find a healthcare professional who specializes in dystonia?  

Look for Neurologist. Dystonia is a movement disorder with abnormal muscle contractions that lead to spasm and abnormal postures. There are many different types of dystonias and various causes for each one. A neurologist is the best place to start since it is primarily a neurological disorder which is causing the contractions. Best of luck.
Ask your MD. You will need to go through your primary doctor. He can find an appropriate specialist who can be authorized through your insurance coverage.