I haven’t tried a drop system yet, but I have attached a Mantis claw to my Inspire to pick up objects. One thing I can tell you is it can be hard to control a drone with a heavy load attached on a string below (swinging back and forth). So, that’s something that could be an issue depending on what you’re planning on attaching.
DJI doesn't sell accessories like this since their drones were not designed to carry extra weight beneath the drone. It's of course something that will void your DJI warranty if it causes the drone to crash.
This drop system seems to be the most commonly used drop system. It can be found on many different websites – and it’s resold under many different names. You can find a lot of reviews for it here on Amazon.