I found them easily enough at my local supermarket, at 128 yen a pop.
I heard the fruit is called Sumo in America because it looks like it has a little top knot.
Chad and I cut open the fruit, took a bite, and....
...it tasted like an orange. Like a mixture of an especially sweet orange and a slightly tart mandarin orange. Personally, since dekopon is a bit expensive, I'll probably just stick with oranges in the future. But I'm still glad I tried something new!