How do get a dubious disk in Pokemon HeartGold?

Answer 1

go into the right entrance of mt. mortar on route 42. when you get in, on your right there will be a wall to use rock climb( hm get off prof oak. on and once you're up, walk downwards and go down the stairs keep going on your left and go through the exit. on your right will be another wall that requires rock climb hm go down and theres an item on the ground. that's dubious disk