We advise that your parents should apply for nonimmigrant visa type "O" from Thai Embassy / Consulate oversea. After they enter Thailand they can stay in Thailand for 90 days. If they need to stay longer, they must extend as the dependent visa, which will be expire the same as yours.
** Please note that if they extend the dependent visa in Thailand they must have the document showing the relationship between you and your parents such as your birth certificate or letter from the Embassy of Korea. (In case that the document is not in English, you must translate to English and certify by the Embassy).
Due to the Non-O Visa must be sponsor by Adecco and we normally charge to you directly. The normal price would be approximately THB 25,000 per application.