Purple sweet potatoes are often available in Japanese markets, or stores catering to an Asian or Pacific Island clientele. If you can't find any, ask your grocery store to special-order a box, and split with friends and family - you'll plow through these tasty little purple treats at a rapid clip. Also available online
. Mine were $1.89/lb at Farmer Joe's
in Oakland, CA.
Note that these potatoes are grey/white on the outside, and purple on the inside. Other varieties of purple sweet potato have a reddish-browninsh-purple exterior, but tend to be a bit dry for making sweet potato balls.
Wash the exterior of your sweet potatoes, place them on a foil-covered baking sheet, and stick in a 350F oven for 1-2 hours (depending on potato size) until thoroughly roasted. You'll be able to squish them with your hands just like an orange sweet potato when they're cooked through.