Cactus sap is harvested from cacti seen around the map. The smaller cacti can be harvested with hands or, more efficiently, using a Whip. The larger ones require a tool such as a Pick or Hatchet - with the Chainsaw being the most efficient.
On Extinction, cacti can be found in the Desert Dome.
In Genesis: Part 1, frozen cacti can be found in the Arctic Biome and can be harvested for sap.
The following creatures reduce the weight of Cactus Sap by the listed amount while it is in their inventory: