My wife wants to apply for S1 visa from Pakistan.

She needs invitation letter of duly authorized entity. I am working in Taixing, Jiangsu province. I have asked Taixing foreign affair office they say they dont know about this invitation letter and they have never done this before. Then i have asked Taizhou and Nanjing foreign affair, they told me the same thing.
I am very worried what i do. Can anyone help me what should i do, how can i get this letter.