Specifically:
Venom Knife
[Ammunition]
Item ID# 1771 (Venom_Knife)
Type Ammunition
Class Throwing dagger
Buy 50z
Sell 25z
Weight 0.5
Attack 30
Required Lvl None
Weapon Lvl n/a
Property Neutral
Applicable Jobs Assassin
Description A throwing knife that has venomous properties.
When thrown at enemies, there is a possibility it will Poison them.
Item Script {},{},{}
Dropped By No Result

Actually looks like

thanks

1 Knife [3]
1 Sticky Poison

I think that's fair.

And many bodies of dead snakes!

Denied: Venom Knife can be bought in game. There is an NPC on map in_moc_16 22 20

Check there and you can purchase them.