Where do you get the rice cakes?

You need to find an Oriental food store near you, preferably one that specializes in Korean foods. They sell them either frozen, or sometimes fresh. You can also order them through places online, like Amazon. If you feel particularly adventurous, you can even make your own, with a recipe from Maangchi at
https://www.maangchi.com/recipe/garaetteok

I recommend buying or ordering them.