Question

Where do you get Razor Fang in Black?

Answer 1

Route 13 - Treasure Hunter (Random item, once a day) Could be a Razor Fang.

Abundant Shrine - Southwest of Lass Maki. She uses a Burmy (PC), Tangela, and a Sawsbuck (so you can find or remember her)

Battle Subway - It costs 48 Battle Points

Answer 2

Go to Route 13. There is a treasure hunter that gives you random items: one of them being Razor Fang. There's only a 1/27 chance, however.