How do you get the black team rocket uniform in pokemon heartgold?

You have to defeat the first 7 Gym Leaders in Johto, you'll receive a call on your Pokégear after you do all of that saying there's an issue with the radio waves. You head to the Goldenrod City Radio Tower and you'll find that as soon as you go in there, it's crawling with Team Rocket Grunts. If you talk to the Grunt blocking the staircase, he'll tell you he's under orders to not allow anyone up the stairs unless they're wearing their uniform so you have to go into the Underground Tunnel which is nearby and head up to the north part of the Underground Tunnel and talk to the Team Rocket Grunt there. He'll put you inside a Team Rocket uniform however this is only a temporary thing.