invitation letter from authorized unit in china

Lan, this invitation letter is issued by a unit in China authorized by the Foreign Affairs Ministry. It's not required for every applicant. So you may not need it. Anyway, in case you need one, you should ask the company or invidual in PRC to help you to apply for it at the municipal or provincial Foreign Affairs Office. It's troublesome and time-consuming but not too difficult to get it. Of course, it's more difficult for an individual to get it.