from Prepaid Data SIM Card Wiki: Korea has an excellent network of open and commercial ?WiFis.
from changwoo[m]: non-korean data esim providers sell much cheaper plans. If you don't need a local number, I don't recommend korean carriers.
StefanoRivera used an international eSIM for a day, before giving up and buying a local one, because it wouldn't stay connected. This was a ?KeepGo Tucana. YMMV
Local plans
If you want a SIM card (Or eSIM) from Korea local carrier, you can get one from one of the below. All 3 are available from stalls in the airport arrivals hall, and usually come with a free TMoney public transit card.
SK Telecom
https://www.skroaming.com/ 20Mbps Unlimited
KT
https://roaming.kt.com/ 3GB / 5Mbps
LG U+
https://www.lguplus.com/ib-roaming/pc/eng 3GB / 5Mbps