How do you get the aura sphere TM in Pokemon diamond?

You are not able to get Aura Sphere as a TM unless you hack. If you can't/do not want to hack then you will be still be able to get it the harder way. You can get any legendary (discluding Arceus) and level it up to level 100. When it gets to level 100 it will learn Aura Sphere. Lucario learns it at Level 34.

Palkia, Dialga, and Giratina learn aura sphere at level 90.