Apply for a Visa at a Border Post
If your stay in U.S. is valid, you do not need a visa to reenter from Canada or Mexico An I -94 is sufficient.

Exchange Vistors
Official document from US Department of State.

J Exchange Program
J visa information.

Re-entry Permit
Re-entry permit for travel after you obtain GC

The American Embassy in China
Includes visa info, US citizen services, trade & commerce, and links of Consulates in China.

US Consulates
US consulates around the world

Visa Appointment Reservation System
Online system for booking non-immigrant visa interviews.