IT IS NOT IN POKEMON DIAMOND OR PEARL BECAUSE IT IS ONE OF THE STARTING POKEMON IN LEAFGREEN!!!!!
Migrate one from firered or leafgreen.
You cannot find bulbasaur on Pokemon pearl, diamond and platinum but you can transfer from GBA via pal park.
You can't find Bulbasaur in the wild on Pokemon Diamond or Pearl (not sure about Platinum).
You cannot catch bulbasaur in diamond and pearl. If you have firered or leafgreen can set 6 Pokemon free in pal park and catch them there.
Migrate it.
there are 150 kinds of Pokemon that you can see and/or obtain. Unless you have friend trade you Pokemon that you can only get in pearl to you, you'll never obtain them all. In Pokemon diamond and pearl all you have to do is see the Pokemon and you get its data unlike the older versions where you have to obtain them.
darkrai and arcus
if you want Entei in Pokemon Diamond/Pearl you'll need a Pokemon FireRed/ LeafGreen that you started out with Bulbasaur. In FireRed/ LeafGreen if you started out with Bulbasaur [Grass-Poison] after you defeated the Pokemon and if you run into Entei in the wild capture it and migrate it into to Pokemon Diamond/Pearl through Pal Park. If you want to migrate in Diamond/Pearl you'll need to see all 150 Pokemon yes both Dialga and Palkia. You'll see Dialga in Pearl from an elderly woman who's shows you its picture and the same with Palkia in Diamond from the same elderly woman in Celestic Town. This adds its data to your Pokedex and less Pokemon to see in Diamond/Pearl.
Milktank is located in route 209 and 210 in both Diamond and Pearl.
No, you do not need diamond and pearl to obtain all the Pokemon in the national pokedex. You either need to have a friend who has all the older Pokemon, or you could transfer the Pokemon yourself from older games.
You cannot capture Bulbasaur in Diamond or Pearl (not sure about Platinum).
Bonsly is located in the trophy garden in Diamond and Pearl as well as routes 209 and 210 for Pearl.
You must MIGRATE!!!!!!!!!!!!!!!!!!!!!!