CMusic Studio is located near Dujeong Station (두정역, line 1) , which is just south of Seoul in Gyeonggi-do.
They seem to have a lot of teachers for keys, drums, guitar, etc, for various types of music styles, but I'm sure you can just rent the rooms for your private use as well. I couldn't find the information that says how much the rooms cost, etc, but you can either have a Korean friend call for you, or call the BBB (Before Babel Brigade), and they will interpret for you for free: 1588-5644.

Here's the map to get there. They're located on the 3rd floor:
Their website is www.cmusic.kr, and their number is 041-563-7177.