From Bulbapedia, the community-driven Pokémon encyclopedia.
Now that the main game is over, it is time to begin the post-game. You'll start at the place where the game originally began, your bedroom in Twinleaf Town. When you go downstairs your mother will mention that Barry was just over, and mentioned something about catching a ship at Snowpoint City.
By now you should have all 210 Pokémon sighted and by traveling to Sandgem Town you can get the National Pokédex. If not you may have missed some. Besides facing every trainer during the main game there are a few that fall outside this as mentioned earlier. Here is a list of regional Pokémon not found in trainer battles excluding Giratina that can help deal with this issue.
- Rotom:Found in its spot in the TV at the Old Chateau but only during the night.
- Unown:Found in the Solaceon Ruins.
- Manaphy:Picture found in a book at Mr. Backlot's office in his mansion on Route 212 which is located on his desk.
- Mesprit, Azelf, and Uxie: Found in their caves at their respective lakes after the Giratina battle in the Distortion World.
- Dialga and Palkia: Counted as seen during the story event at Spear Pillar.
If you are ready head to Sandgem Town.
Once Professor Rowan confirms you have seen all the Pokémon in the Sinnoh Dex, Professor Oak will appear. Having heard of your exploits, Professor Oak will reveal he is bringing components for the National Pokédex and your Pokédex is automatically upgraded. Before he leaves, he'll mention Pal Park on Route 221 is open which will allow you to import Pokémon from other regions.
Professor Rowan will then give you the Poké Radar that will allow you to find non-regional Pokémon in grassy areas. All you need to do is stand on a grassy area without being mounted on your bike and like the VS. Seeker it requires 100 steps to recharge it.
The house below you is Lucas/Dawn's house. Their little sister can tell you which of the swarm Pokémon is present today and it changes daily.
| Available Pokémon (Swarms)
Once you are inside then you'll meet Professor Oak who will mention that Pal Park can import Pokémon from Hoenn and Kanto before he gives you the Trainer Counter app for your Pokétch and then leaves for Eterna City. Now while you can import Pokémon from Pokémon Ruby, Sapphire, Emerald, FireRed, and LeafGreen, though you can only do it with a Nintendo DS or DS Lite in addition to these Generation III game cartridges.
Now while Pokémon HeartGold and SoulSilver can help with the Pokémon not found in this game or Diamond and Pearl, the only ones that need to be imported from Generation III are Regirock, Regice, or Registeel since they cannot be found in the Generation IV games unless if you have the special event Regigigas for collecting them in this game.
Like with Pal Park by using a Nintendo DS or DS Lite with a Generation III Cartridge installed in the GBA slot it allows for certain Pokémon to appear on certain routes and areas throughout the game with the Dual-slot mode feature. The non-regional Pokémon that appear specifically based on which game cartridge you are using can only be found in the wild through this method. Without this feature, the only other alternative is to collect them in both HeartGold and SoulSilver and then trade them over to Platinum.
Professor Oak is located in the house next to the gym in the southern part of town. Talk to him and he will give you another Upgrade and will mention that Kanto's Legendary birds Zapdos, Articuno, and Moltres are now roaming around Sinnoh.
Talk to Mr. Backlot and he will mention of a particular Pokémon in his garden. This changes daily so check back frequently. Occasionally it is a non-regional Pokémon. See this section for the Special Pokémon that appear.