After the elite four, you will have seen most of the Pokemon if you battle every trainer you see. Here are some Pokemon hard to find in sinnoh...
Riolu:Hatch egg from Riley or go to the Oreburgh gate, after you get the bicycle from Eterna City, and go up. you will need surf and strength to do this so at least after the canalave gym you should use this plan. Anyway, go on the lake and battle the vetaran on the other side, to the west. He will have a Riolu.
Lumineon: On the route to the victory road, one of the girls will have a lumineon.
Combee: On the route to the west of Hearthome City, there will be an aroma lady who is the only trainer who will have a combee.
Dialga(Pearl)/Palkia(Diamond): Talk to Cynthias grandma after beating the champion in the big house to the north.She will show you the opposite of your game cover.
That's it from me!!! If you have more Pokemon answer and that will help!
you have to get the national dex when you see all 150 Pokemon orif you play platinum you will have to see all 210 Pokemon
Well when you SEE all 150 Pokemon prof. Oak will give you a deploma then go to cinnibar island and head inside and talk to bill...and be ready for an adventure!! Then when you own all 150 Pokemon you can get the mystic ticket! (which you can only get in an event) this allows you to get ho-oh and lugia! yipee
See ALL 150 Pokemon in the Sinnoh Dex, then go to anyPokemon center and access the PC. Go on to "Rowans PC" and he will give you the National Dex.You have to see all 150 Pokemon to do that u must beat the elite four.First of all it depends of what game your playing mostly you will need to register over 60 Pokemon or more in your pokedex then show it to the professor in that game, if that's not the case you may need to see all of the Pokemon in that particular region.You need to complete the game by beating the Elite Four and your rival and see all the Pokemon in the local dex once, then you can get the National Dex.
150 hope i helped :) Actually there are 151 pokemon in the Sinnoh Pokedex but you only have to see 150 to get the National Pokedex.
Here is a list of all 150 Pokemon: http://www.serebii.net/diamondpearl/shinoudex.shtml The last Pokemon listed (#151) is not required to get the national pokedex.
you have to get the national dex when you see all 150 Pokemon orif you play platinum you will have to see all 210 Pokemon
By Trading.
you have to see all the Pokemon through 150 then talk to professor oak
See all 150 Sinnoh Pokemon
You have to see all 150 Pokemon, you don't have to catch them. After that go to Professor Rowan
you have to see all 150 Pokemon in the sinnoh poekdex
You can get the National Pokedex in Diamond when you see all 150 Pokemon, but not necessarily capture them.
See all 150 Pokemon in the Sinnoh Dex and talk to Professor Rowan.
you have to see all Pokemon in the game (sinoh only) and the go to see rowan the profecer and oak will appear
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.
No all you have to do is SEE the first 150 Pokemon then you shall get the national dex..
all you have to do is see about 150 Pokemon is all and make sure they are all different (and try to catch them all because some are very rare and you can only see them once)