I tend to buy daifuku manju (大福まんじゅう) with a plain cake and a sweet red bean paste filling (pictured above, sometimes the rice gluten cake is white instead of pink so read the label!).
However, the rice cakes also come in other flavors, including macadamia nut (white, below at left) and mango (orange, below at right).
The macadamia nut cake was unexpectedly crunchy, due to chunks of macadamia nut in the cake's exterior. The filling was my favorite - red bean paste. The mango cake, with a mango-esque filling, was a bit cloying for my taste.
A Japanese confection, rice cakes can be found at Asian groceries. In Denver, I've found them at the Pacific Mercantile and at H-Mart. Sometimes they are in a refrigerated section (Pacific Mercantile), sometimes in the freezer (H-Mart), and other times they are on the shelf, or even by the cash register. I'm not aware of any reason why the rice cakes need to be refrigerated or frozen; they don't last long in my home, anyway.
Thanks to Deb in Denver for the Japanese translation.
Pacific Mercantile Company
1925 Lawrence
Denver, CO 80202
H-Mart
2751 South Parker Road (Parker & Yale)
Aurora, CO 80014