The source of problems is Ulrich Görtz great site http://www.u-go.net/classic/. He has a huge collection of go problems, A invaluable source for any go player training, including me :-).
Hyun Hyun Ki Kyung is the Korean name of the collection called Gengen Gokyo in Japanese resp. Xuan Xuan Qi Jing in Chinese. It contains 347 problems with solutions.
To get next problem press the Next button in java applet