What are you wanting to use it for?
A rabbit isn't going to go for a walk like a dog. Rabbits tend to run forward, stop and dart off again. This can make using a harness dangerous. If you are not paying attention, the rabbit can snap back. They can also get tangled in the leash, so it needs to be help up off the ground. It's not that a rabbit can't be on a harness and leash, but you need to pay close attention to the rabbit and the surroundings (dogs, cats, people, cars etc, can all scare a rabbit). An x-pen would be a better way to have your rabbit outside if possible.
I do agility with my rabbits and use a harness for that. It is really to have something to hold onto and grab if needed. We don't use leashes other than for training and if the area is not 100% secure (usually use a bunch of x-pens connected to go around the course), of if we are on grass as the rabbits don't focus well, otherwise the run off leash. One of the biggest reasons for not allowing leashes is that people don't know how to use them, drop the leash and it gets tangled around equipment.
If you are going to use a harness, you need to make sure to get a safe one. I recommend and H style harness. Some types like Figure 8 ones, can tighten when on the rabbit and strangle them. Vest harnesses are ok, but can be hard to get tight enough, the velcro gets clogged with fur, they are bulky which can lead to overheating, and the velcro can also stick to other things (I have heard of some rabbits getting stuck to the carpet).
This is an H harness.