Rice cakes are a stigmatized food in the U.S. They're the food a dieter forces herself to eat as a "healthy replacement" for chips or cookies. In Korea rice cakes seem to be a popular snack food. Street vendors sell them. You can hear the characteristic "thwack!" as a new rice cake pops out of the machine. E-mart sells them too: Surprisingly, no one is sampling the rice cakes. Usually you'll find several aggressive older Korean ladies huddled around a sample plate. Sometimes it seems they are eating a meal, not sampling.
Anyway, you can find plenty of variations, from grain type to size: