you go to the underground tunnel in golden rod city then you talk to the team rocket guy down there and he will give you a suit. then go to the radio tower and thee they are.
go to the underground part where the guy takes pictures of your Pokemon for you and wen you talk to him he'll ask you if you wanna dress up as a team rocket person. hope this helps :)
If you're playing Pokemon Gold or Silver (or Crystal, maybe) you must return to Mahogany Town and go east. Or you may go to the Tin Tower in Ecruteak City and catch the legendary Ho-Oh.