Ultra Thin GPS tracking devices

Our business is looking to find or create small tracking devices to on put on all our items, so we can track if they are stolen, missing and/ or how far they are away from delivery. These items sizes vary from very large to as small as the palm of your hand. We are looking to get an extremely thin 'gps tracking device' or similar tracking devices that will allow us to put onto each item so we can track them all over the country. The device can be as wide or as long as needed but must be thin (less than 3mm). They need to work for at least 7 days each. They dont need to look great, we can tape over or hide, just need to work effectively. Once the first product is developed/ sourced all additional products need to be able to be produced cheaply as we will need thousands of them if they work well. Thanks so much, hopefully someone can assist!