Mine have loc8tor trackers on their collars, in the tag cases which slide over the collar. They are far from perfect, not least because the range is quite limited, but they do give me peace of mind. They are supposed to work over 400ft+ in an open space, but it's nothing like that through fences and walls. Even so, if you walk around your area for long enough waving it in all directions, you are likely to find them eventually, which is why I stick with it. If your boys lose their collars a lot that isn't too helpful, but on the up side, you will at least be able to find the collar again! And if kitty goes missing when collar comes off, it will give you a clue as to which way they went.
I haven't seen a proper GPS suitable for cats yet, because they require a constant power source, the battery packs make them quite large so at the moment they're only recommended for dogs. Give it another 5 years or so and I reckon GPS for cats might be an option.
