He's in the Great Marsh
sudowoodo, croagunk, and happiny.
the symbol Pokemon of pastoria is croagunk as shown in the Pokemon episode diamond and pearl #082 cream of the craogunk crop. since croagunk is its symbol they have a sign in pastoria to help you feel as awesome as croagunk by looking like it through the sign in platinum.
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
Milktank is located in route 209 and 210 in both Diamond and Pearl.
Croconaw evolves at level 38, not by a stone.
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.
IT IS NOT IN POKEMON DIAMOND OR PEARL BECAUSE IT IS ONE OF THE STARTING POKEMON IN LEAFGREEN!!!!! Migrate one from firered or leafgreen.
That's easy, croagunk evolves at level 37-38.
Bonsly is located in the trophy garden in Diamond and Pearl as well as routes 209 and 210 for Pearl.
You go to Fuego Ironworks.