Is there any Japanese supermarkets or department stores in your area?
(Ito Yokado and JUSCO, for example, have some stores in China.)
If yes, it's worthwhile to ask them if they carry any Japanese food including natto.
Famous Japanese brands for natto are Okame (by Takano) and Mitsukan (do not know what brand name they might have in China - in Japan they are "niowanatto", "hone-genki", and "kin-no-tsubu").
Ordinary natto should not be available on the net as it is perishable food.
If you want something from net, then you should look for dried natto Dogen mentioned.