They can be anywhere on your island beaches, however if you walk around a certain area repeatedly they will show up if you’d rather not traverse your whole island to find them. It’s random spawns, like normal manilla clams.
In subreddit r/AnimalCrossing
In a post titled:
Hello friends! Does anyone know how can I get Gullivers communicators? Ive been trying to find it in the ocean and sand but still no luck for the past 3 days looking for a new horizon friend too, message me if interested!