When you arrive at Pal Park for the first time, Professor Oak gives you the PokéRadar and Pokétch feature 20. The PokéRadar allows you to guarantee a Wild Pokémon appearence. However with the National Dex in your possession, you will be able to find Pokémon that aren't in the Sinnoh Dex. The way the PokéRadar works is that when you use it in Grass, without your bike, you will see the grass start to rustle. Go to it and you will have a Wild Pokémon encounter. However this does not guarantee that you battle the old Pokémon, you can battle any Pokémon in the route.
However you do not have to worry if you find an old one, if you battle the Pokémon and accidentally faint it, you will get another chance at it when the PokéRadar goes off again immediately after battle for a good few times. This also helps you to get a specific Gender if for example you are hunting for a Male Kirlia or a Female Snorunt for evolutionary purposes.
Function 20 of the Pokétch also works utilising this. It displays the biggest streak you have had battling Wild Pokémon found with the PokéRadar, just for bragging rights.
A feature that was introduced in Past games is the implementation of Swarms. These are where Pokémon in certain routes have their appearence Percentage raised so that they are more common.
In Diamond & Pearl however, after you get the National Pokédex, if you go to your Other Rival's house in Sandgem Town and talk to their sister, they will tell you that there is an increase of Pokémon in a certain Route or area
If you go to the route or area that the girl states, within one or two wild Pokémon battles you will find the Pokémon at the same level as the other Pokémon.
The Pokémon and Route change every day and all of the Pokémon that you find are not obtainable by any other means in Diamond & Pearl so be sure to capture them while you have the chance.
In Route 212, there is a Mansion where a few people live. Out back there is a patch of grass which is predominantly inhabited by Roselia, Mukubird and Kricketune. However if you talk to the owner in the room to the right, you can activate something else.
Every day if you talk to him, he'll walk out. Go back in and he'll be there and tell you of a Pokémon that is inhabiting the place today. Sometimes its a Rare Sinnoh Dex Pokémon, others its a Non-Sinnoh Dex Pokémon so be alert. They are however there all day so you can go back. Their level averages at Lv. 16 to Lv. 19 so bring a Pokémon of that level.
You do need to activate this event every day if you wish to see the Pokémon
The final method that allows you to obtain non-Sinnoh Pokémon involves the GBA Games. With Diamond & Pearl being on the DS which contains a slot for GBA games, there is an extra feature utilising this. When you have certain Pokémon games in your GBA slot, some new Wild Pokémon appear in certain areas for the period of having the respective game in the GBA Slot. However these areas do not appear in the Pokédex so this page will be used to help you with the Pokémon Locations for each of the games. These Pokémon and their evolutions are only available with this method
However this is not the only reason to find them. Attached to a small amount of the Wild Elekid & Magby are the ElectroBooster and MagmaBooster which are used to evolve Electabuzz and Magmar into the final forms Electivire and Magmortar so this is necessary to hunt the Pokémon down using this
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The Safari Game also houses many old Pokémon outside of the Sinnoh Dex. Once you have the National Dex, if you go up to the binoculars in the Safari Game and have a look, you may find an old Pokémon.
However this does not mean they will definitely be there as they will be pretty hard to find in that locations so you have to be patient. Many old Pokémon are only found here so be vigilant
Generally Wild in Sinnoh
A good deal of Pokémon will be found in the Wild Normally on Routes 224, 225, 226, 227, 228, 229 & 230 as well as in Stark Mountain and a few other places that are only accessible after you get the National Dex. Since they are wild and not time dependant, you do not have to worry about getting them if you see them, you can be picky with them.
Also, out and about in Sinnoh normally now, you will be able to obtain many old Pokémon simply by fishing