SIM is pretty much a GSM concept. It's technologically possible to have a SIM on CDMA, but it isn't done, mostly because CDMA phones generally can't move between carriers, so there's no need to change SIMs.
There are prepaid phones in Korea, but they don't use SIMs, and they will work only in Korea. Both the phone and the service are expensive. I can't remember specifics since I priced this out, but a basic phone was over $300, and service itself was also more expensive than elsewhere.