they are the ones with a white skin and when you cut them in half they are bright purple. you can google them and read about them but i cant find anywhere where i can buy seeds, slips, or the tubers themselves.
where can i get a purple okinawan sweet potato?
They have them at this place, but state that they are very poor sprouters. Good luck.
http://www.sandhillpreservation.com/catalog/sweet_potatoes.html