I'm not entirely sure what you mean by your question of a "card that can load", because none of the KR Nexon games to my knowledge are pay-to-play; you need a KSSN to get an account (plus a phone number verification if you're using an adult KSSN), but other than that, it's completely free.
Unless you mean cash cards. In that case, they're available in almost literally every single convenience store you can find in Korea, because MMO gaming is just such a hit there. My personal favourite is CultureLand. However, you can save yourself a visit and just buy them from online retailers such as CN-USA.