Saw your questions about the rejected FP posted days ago. Not sure if you got the answer. In the rfe, there should be a list of the agencies that you can get the police clearance/certification with their phone numbers and address. Call the ones in the states where you lived in the past 5 years. They will tell you how to get to their web and print the request form. Complete the form and send with the required the fees. You will then receive the documents from them. The uscis will not ask you to do the fp again after rejection twice, but the rfe for the police documents instead.