You can only get another starter by trading with another game in the same generation.
The starter pokemon for Diamond and Pearl are Turtwig, Piplup, and Chimchar.
you can get it as your starter Pokemon
Chimchar
Turtwig Chimchar Piplup
Turtwig (grass) Chimchar (fire) Piplup (water)
they are chimchar, piplup and turtwig
Chimchar is on of the starter Pokemon in diamond. As far as I know, you can't catch a chimchar in the game, but if you want one and didn't choose chimchar as a stater Pokemon, you could trade with someone for a chimchar of WFC.
piplup, turtwig and chimchar, same as diamond and pearl
Fire: chimchar Water: piplup Grass: turtwig
the best Pokemon to pick at the start is PIPLUP or CHIMCHAR
in order from best starter to worse Piplup Turtwig Chimchar
turtwig,piplup,and chimchar are diomonds three starting pokemon