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It's not our policy to recommend specific stores or sellers, but try the local privately owned plant stores and nurseries in your area. Often they carry plants the Big Box Stores don't, or will order for you, or give you advice as to how to find. Also you can google the plant, which will probably turn up online sources.

It sounds like it could be white rust, edema or a fungus of some type. Here is more information: https://www.gardeningknowhow.com/plant-problems/disease/white-rust-disease.htm (uncommon in ornamental sweet potatoes but not unheard of), https://www.gardeningknowhow.com/edible/vegetables/sweet-potato/sweet-potato-with-white-leaves.htm It may help to treat the plant with a fungicide, like neem oil, to treat any fugal issues or even insect pests that may be affecting it.

Typically blights on sweet potatoes (Kumara for you there) is caused by bacteria and there are no cures for most. I would pull the affected plants ASAP before it spreads to the other plants. If you can give us more detail about the symptoms the plants have, we will be better able to pinpoint the exact blight affecting them.

This article will help: https://www.gardeningknowhow.com/edible/vegetables/sweet-potato/harvesting-sweet-potatoes.htm

Hang them in a warm dry place to finish curing, maybe another week? Maybe a little more. This article has some more information: https://www.gardeningknowhow.com/edible/vegetables/sweet-potato/harvesting-sweet-potatoes.htm

Yes, it grows here just like other sweet potatoes. The reason it is hard to find is simply a supply and demand issue. There is not a lot of demand (because it is relatively unknown in the U.S.), so many large companies do not carry it. Look for specialty sellers who specialize in Asian vegetables or even look on eBay (I have great luck finding unusual and hard to find seeds and plants on eBay).

I found this company that sells purple sweet potato slips: http://www.sandhillpreservation.com/catalog/sweet_potatoes.html

I believe that the reason they must be started from slips is because the tubers (i.e. the sweet potato) do not produce roots and shoots the way white potatoes can.