There are two main ways of getting an Eevee in Pokémon Diamond beyond trading:
1. Obtain Eevee from Bebe after defeating the Elite Four. She will give you one if you talk to her.
2. If you talk to Backlot at the Pokémon Mansion on route 212, you have a small chance of having an Eevee swarm there. It's sort of random though between 16 different Pokémon (I believe).
As for Ditto, they are found on Route 218, and you have to be using the Poké Radar in order to find them. I personally transferred one over from my LeafGreen instead of finding it myself.
Hope that helps.
Gibby