I assume it probably works the same as chicken harnesses. You can use pretty much any harness that will fit around them comfortably. Don't put the harness around the legs, but around the wing arms. Put on strap infront of the wings and one strap behind the wings around the body and it should stay on.
I use a figure eight cat harness for my birds. I also make and sell the chicken harnesses, which would work great for young emu. They do make dog harnesses that are fairly large and could fit them as they grow.
When they are taught as babies to lead on a harness, they will lead gently on a harness as adults and it makes them easier to move around.