In most cases your GP will suggest a medical specialist in a nearby clinic, when he is referring you. You are free, however, to select another medical specialist if you prefer to do so. Make sure that your health insurance provider covers the services of this medical specialist, especailly if you have an in-kind policy.
You may locate a medical specialist through this page or check this external service. It is as yet limited in the number of specialisms available, but this is expected to increase.