You'd better apply for China visa in Thailand, it is complex to get a China visa in South Korea for non-Korean applicants. the Alien Registration Card with at least 6 months validity is required by China embassy in Seoul and you have to apply for visa through one of the travel agencies designated by China embassy in Seoul by paying extra service fee.

China consulate in Busan doesn't asks for the Alien Registration Card with at least 6 months validity, but you also need to apply for visa through the travel agencies designated by this consulate.