Only webshop for Korean singles I’m aware of is tcgbox.co.kr. They only ship within South Korea, but you can use the middleman website harum.io for worldwide shipping. I’ve ordered like this three times thus far without a problem. (PS: International shipping from South Korea was pretty expensive a few months back since only UPS/FedEx/DHL were available due to the pandemic, which are a bit more expensive than standard international shipping.)
Alternatively, for non-Holo or Holofoil Rare set cards I usually just buy the booster box on eBay for 15-25 bucks to pull myself. For Ultra/Secret Rares; Reverse Holos (if they are in the set, which isn’t always the case); and promos, it’s a different story. If they are out of stock on tcgbox.co.kr your best bet is to find someone living in South Korea to help you out. If you or anyone else know about another Korean webshop that sells singles, lmk though. I’m still missing two Korean Pikachu cards to have all of them unfortunately. 
Greetz,
Quuador