Where can you find Tm 80 in Pokemon Diamond?

You can't find TM80, Rock Slide, in Pokemon Diamond. This TM was introduced in Pokemon Black and White.

However, you can learn Rock Slide through other means in Pokemon Diamond:

* Level up: Some Rock-type Pokemon learn Rock Slide by leveling up, like Garchomp.

* Move Tutor: You can purchase Rock Slide from the Move Tutor in Eterna City for a certain amount of money.

Keep in mind that you can only use the Move Tutor on Pokemon that have already learned a move from that specific Move Tutor.