Unlike plastic or nylon loofahs, konjac loofahs are entirely made out of konjac plants that are molded into different shapes as they are dried out. This means it is made of only natural materials and is completely biodegradable once you're through using it.
Konjac loofahs have an interesting consistency of cool, soft rubber. However, it is surprisingly absorbant and can hold a lot of water and soap bubbles. It also has none of the abrasivness of non-biodegradable loofahs, but many online reviews swear that it still exofoliates the skin very well. It doesn't make much of a difference to me anyway, since I periodically exfoliate my skin using either my own exolfiant made of coffee grounds or a baking soda paste.
If you don't plan on using a konjac loofah that has already been taken out of the package for an extended period of time, the manufacturer also advises that you put it in a cool, dry place or the refrigerator to keep it "fresh," and to prevent the risk of mold developing.
Cosmed (康是美) stores in Taipei do sell konjac loofahs, but they usually only have much smaller sizes used for facial exfoliation in stock. Online retailer momoshop (momo 富邦購物網) also sells them, just type in "蒟蒻洗臉綿" into the search bar and a bunch of results should show up. However, there are no English options, so if you can't read the text, you might need a Mandarin-speaking friend to help you with the check-out process.
Unfortunately, I haven't been able to find konjac loofas in the States, but sea sponges harvested eco-consciously are a good alternative.