I have a tourist and business visa to the United States

You can try to apply it through the local nearby embassy/consulate, but it is uncertain that you can get it in the end.

In general, you need to prepare your valid passport, an application form with a recent taken photo, your proof of legal status in USA, your Invitation Letter of Duly Authorized Entity from China, an invitation letter from the fair, your bank statement and other documents the embassy/consulate requires.