I saw some of these for sale this spring at an oriental grocery in Nashville TN. I bought a small one to sprout, but it rotted rather instead. Then I read in VEGETABLES, AMARANTH TO ZUCCHINI that the oriental sweet potatoes are often treated with a sprout inhibitor. Next visit to the store, there were no more purple ones, but I bought a tan-skinned white fleshed one, and it sprouted just fine. The white fleshed ones are more traditional in the south anyway, it probably wasn't shipped from as far away. I did get some yellow sweet potatoes from a neighbor in TN years ago that had some purple veins running in them. I think they were from a strain that had been grown for a long time and had done some mutating. The deep orange strains have to be rouged every few years as they want to sport back to yellow.

The sweet potato you are looking for is called UBE. (pronounced oo-bey)
Good luck finding it.
You may want to check Filipino stores as that is what they make desserts out of.