What you envision isn't practical.Yes, they do have harnesses for adult chickens, but they are meant to control a suddenly flappy chicken in a setting where you don't want an out-of control chicken. They aren't meant as "chicken walking" devices.
Chicks double in size each week, so a baby chick harness, if they even existed, would be hard to keep fitted on a little chick.
However, I understand that you want to be able to control your chicken when you take it out of its chicken coop. Chickens are very easily trained, and it doesn't take a lot of time or experience. You can start training the first week you have a chick. (I hope this chick will have some friends and will not be alone.)
All you need to do to get a chick or chicken to come to you when you call them is to offer treats and say a word or use a device such as a clicker (found at
PetSmart for $1) or a rattle as you show the chicks the treats. Very soon they will come running when you say the word or make the special noise. They remember this training for the rest of their lives.
But a chicken isn't like a dog. Chickens aren't especially interested in being obedient and pleasing you, and they are more inclined than dogs to have their own agenda when you take them out into a field full of grass and wonderful bugs. They tend to completely ignore their human when they are allowed to engage in their favorite activity - scratching for bugs.
OK they haven't done that 
