Very often folk buy them to train up a second or third dog using the same in-ground concealed dog containment system. Many of these systems do allow multiple dog training using the same control box.
I have found five versions of these that will do the job very nicely which have high rating figures from those who use them. A big plus is the fact that you won't have to spend more money having to buy the whole system as you have done previously.
Replacement Petsafe Stay and Play Invisible Fence Receiver Collar
Replacement PetSafe Collar for Dog and Cat Invisible Fence Containment System
PetSafe Waterproof Invisible Fence Stubborn Dog Rechargeable Replacement Collar
High Tech Replacement Multi-function Collar For In-Ground Invisible Fence System
SportDOG In-Ground Invisible Fence Replacement Collar B000FEQZXM