It is hard to get a China work visa within Mainland China, if your employer is famous and reputable in China, such as you work in a famous unversity or a world-famous international comapany, maybe the employer can help you to get a work visa within mainland China, generally speaking, applicants should get the work visa in their home country. The work permit for aliens and visa notification letter are required for work visa application, you can go to China by business visa first, then find a job, ask the employer to prepare the 2 documents for you, then return to USA for work visa application