The place to find cactus sap in the Ark Genesis dlc is in the Artic Biome. That doesn't make a lot of sense, but that's where they grow. They also don't look very obvious, the cactus looks like a spiky purple cucumber. Tree sap in Ark Genesis can be found in the poison trees.
Lat and Long of Cactus Locations
|The Cactus in Ark Genesis