If you want to fill your Sinnoh Dex, look on websites. There are lists of where to see every Sinnoh Pokemon. If you want to fill your National Dex, you have to import them from your GBA games.
yes
Trading.
I believe it is Pokemon Diamond or Pearl
all the Pokemon on route 210 in Pokemon diamond and pearl are ponyta, geodude and if your really lucky you can find onix.
They have all the Sinnoh pokemon, and some more.
No, in Pearl you can't get Skunky, Skuntank or Dialga and in Diamond you can't get Palkia or Sudowoodo
trade diamond from Pokemon diamond onto Pokemon 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.
An action replay DS does work on Pokemon Diamond & Pearl I use it all the TIME!
You trade Magmar from Pokemon pearl,diamond,or platinum to ether Pokemon pearl,diamond,or platinum.
From what i know you complete Pokemon diamond and pearl by getting all the gym badges, beating the elite four and completing the pokedex. But im not sure if you have to catch all the Pokemon.
For Pokemon Diamond or Pokemon pearl The one for Pearl is 12068AC6 000046C0 The one for Diamond is 12068ac6 000046co