On a wild Carvanha, Sharpedo, Clamperl or Huntail. 5% change Source: http://www.serebii.net/itemdex/deepseatooth.shtml
White Forest so you have to have White 2.
Source
You can find them in Route 13 or 17. Wild Red-Striped Basculin and Sharpedo have a 5% chance of holding a DeepSeaTooth. You should use Thief/Covet to steal the item.