Simply go to your local courthouse and file the case in the civil section. usually the prices depend on how much your sueing for. With no proof of payment due such as a bill or agreement in writing between the two of you it could be hard to prove. However unlike a criminal trial you only need a preponderance of the evidence and NOT proof beyond a resonable doubt. Argue your case if you win than good for you, if you lose just chalk it up to business lessons learned. Always make out a bill for every sale so you have the proof just in case you get stiffed. good luck keep us informed.