请教:申请加拿大移民, 如何办理中国无犯罪记录?

According to application form for immigration to Canada, applicant must provide中国无犯罪记录 which is issued within three months. I left China for many years and have no connection at all in China. Does anyone know how could I get中国无犯罪记录? Do I have to go back to China? What about if I could not go back? Thanks a lot.