You can buy purple sweet potatoes from several sources depending on your location:

• Online Retailers:
  • Oriental Webshop offers daily fresh purple sweet potatoes by the kilogram

* Riverford sells organic purple sweet potatoes, available for delivery in the UK

* My Exotic Fruit provides purple sweet potatoes with UK delivery options

* Watts Farms sells purple sweet potatoes by the kilogram online

* Asia Market offers fresh purple sweet potatoes, approximately 1 kg per package

• Grocery Stores and Markets (U.S., specifically Minneapolis area):
  • Trader Joe's, Whole Foods, and United Noodles carry Japanese purple sweet potatoes, some with white interiors

* Seward Co-op in Minneapolis is known to stock both Stokes Purple and Japanese purple sweet potatoes

* Asian grocery stores like Shuang Hur, Dragon Star, and BF Yangs often have purple sweet potatoes

* Farmers markets in Minneapolis during the fall season typically have a good supply of purple sweet potatoes

• Specialty Produce Suppliers:
  • Frieda's Specialty Produce offers Stokes Purple® sweet potatoes, though currently they are not available for direct online purchase but can be found in local grocery stores they supply

If you are outside these regions, checking local Asian markets, health food stores, or ordering from online specialty produce shops is recommended. Availability may vary by season and region.