Where at in the great marsh is defog in Pokemon diamond?

just go into the marsh and talk to the one kid, he'll say he doesn't need it

firstly if you have fly it be much easier to get to pastoria city once you are in that city you go to the safari park called the great march once you go inside talk to the person on the desk and he/she would ask if you wanna enter once you do all that you go outside the other end and you see two guys the one on the right has the defog thanks for your coorperation