To get a Pikachu with Volt Tackle in Diamond, you need a Light Ball first of all.
To get a Light Ball you need to catch a wild Pikachu that is holding a Light Ball.
The only place you can find Pikachu in Diamond is in the Trophy Garden.
But there is only a 5% chance to find a Pikachu holding a Light Ball. So I recommend you get a Pokemon with Compoundeyes, like Venonat or else a Pokemon with Static to attract Pikachu.
Put him in the front of your party. You might want to try and chain the Pikachu cause it might be easier to find him that way. I also recommend you get a Pokemon with Frisk such as Banette so you find out if he is holding a Light Ball or not. Once you encounter one with a Light Ball, catch it, breed it and Voila! You have a Volt Tackle Pichu :D then just evolve him into a Pikachu via happiness and now you have a Volt Tackle Pikachu :D
Hope I helped!