Great question, the best option is to deal with a professional third party administrator (TPA) specializing in management of drug and alcohol testing programs. There is more to drug testing programs then just collection a urine specimen collection.
Learn more about a Consortium/Third Party Administrator (C/TPA). These are the type of companies that can help you manage your drug free workplace and drug testing of your employees. Your local medical clinic can certainly collect urine for drug testing but not typically qualified for management of drug testing, random testing, compliance and drug free workplace policies and procedures.