Purple potatoes / murasaki imo
Ever since I went to Okinawa a few years ago and fell in love with the murasaki imo, I've realized I haven't been able to find the actual potatoes on the mainland. I've seen desserts and other sweets *using* murasaki imo, but I've never actually seen the potatoes themselves for sale. I saw a pamphlet in Kobe Airport that noted murasaki imo was one of the items prohibited by law to bring back to the mainland (!!) because of pest issues... but is there really no way to get them?! :(
Murasaki imo is one of the standard imo variety in Kagoshima, so one can find it abundant in Kyushu. Here in Hiroshima, I've had it included in my grab bag from the CSA (community supported agriculture) farm, so it does grow locally in Honshu as well. I'm pretty sure I've seen it at the markets around town as well. I guess you would have to be down around these parts where imo farming is more prominent.
Every couple weeks, The Tenmaya and Takashimaya near me have food festivals from various parts of Japan, the Hokkaido one just passed through and there has been an Okinawan one as well, and they often have produce from the area, so that might also be possible in the future if you don't have luck at the store near you.