The cameras are found pretty much everywhere in the town. Whoever abducted the seven abductees also placed them there. In Episode 2, we see Janet shoot a camera, and seconds later, a new one appears in its place.
There are also cameras set up in the homes of some of the abductees. There is a camera in the living room of Janet's mother's house as shown in Episode 2. In Episode 3, there is a video shown on a TV in town of Charlie smothering his wife with a pillow in their home. This tells us that there is probably a camera setup in his home as well.