You can find a medical specialist via the Internet, for example at the local site of the federal medical association:
http://www.bundesaerztekammer.de/service/arztsuche/nordrhein-westfalen/
There are local services to make an appointment with medical specialists, for example:
https://www.kvwl.de/patient/terminservice/index.htm
https://www.kvno.de/20patienten/17tss/index.html
Please make sure to have a medical transfer by your family physician.