Instructions
Complete as many missions as possible on the first four levels -- Los Santos, Badlands, San Fierro, Desert -- to unlock the "Las Venturas" level where thermal goggles are located.
Check map for the area marked "Bone County" and make your way to the large "Big Ear" satellite station that looks like it has been designed to listen in on alien signals.
Explore the barren area around the massive satellite dish until you find the "Sandking" off-road vehicle. The thermal goggles are located very close to this 4x4 truck.
Add thermal goggles to your equipment list. You can only use goggles once. And, if you die the goggles will not reappear.