I buy Naga.imo - Long (wild) potatoes — at Hmart. But many of their produce are conventionally grown and treated not to sprout in storage/shipping, so I’m not sure if they will grow. Best if you can get organically grown ones.
According to Japanese website there are THREE major types, the cultivated Nagaimo, Jinenjyo (which appears to be closer to the original wild species), and another type that grows paddle shaped or mouse-antler like roots. Actually, in Japan, it’s these moose antler shaped (or Ginkgo leaf shaped — i.e. Icho.imo) types that are called Yama.imo — Mountain (wild) potatoes.
The Japanese for those “seed potatoes” is “Mukago” ... I’m not sure if this will work for you, but try this google search link
— some of the growing method images may be of interest
Here’s an example of one showing different ways to start them — B and C are recommended.
...hmm... that link has gone defunct, try this — https://www.jinenjyo.net/susume/#saibai_3