Yes, you can usually find them at Incheon Int'l Airport.  Just ask at an information kiosk/booth after you land in Incheon.  If landing at Gimpo Int'l, they should also have this service, but it is usually better to reserve a rental phone in advance.

 

Here is a link that should be helpful in preparing a mobile phone for your homestay in advance:

 

http://english.visitkorea.or.kr/enu/RE/RE_EN_1_2_4_1.jsp