After Lake legendaries, Palkia... where the hell I get the rest?

[Dark-San]

Okay this is nuts I know but I spent the last three days and a half completing in the Battle Tower to exchange for the items that I might need for the tournament. Now it turns out that I can't join due to me overtraining my Pokemons. Great, all of my Pokeys are in their level 70s and all of them are all above 50. >.<" I should have stop training by the time I hit the Ghost gym since it was then that most of the Pokemons are in their level 50 and below. Besides training, I went on a major catching Pokemon mode in order to complete my national dex.

The phrase 'Gotta catch them all!' had became a thing that I would never achieve unless I completed the last few remaining last spots. These missing pokemons are Darkrai, Shaymin, Cresselia, Giratina, Regigigas and Heatran. I could not find them throughout the entire Sinoh region and thought that they might be diamond exclusive.

Does anyone know where can I get them? And do I have to beforehand before they appeared?

[RattataGuy]

You can forget about obtaining Darkrai and Shaymin without cheating. Those require key items and event flags that can only be set at a Nintendo event (or cheat device).

And as for the rest... these are ALL post-E4 pokemon. So if you've not even beaten the game first don't even try (that's like a cardinal rule almost in most pokemon games).

I'm pretty sure someone will give you a full walkthrough after I post this, but for the sake of exploration and adventure, here are some hints.

Cresselia ~ Go to Canalave town... you may want to try to head to Iron Island in fact.

Giratina ~ Sinnoh has a mysterious hidden fourth lake. Look for it. (By the way, this is the 3rd "mascot" apparently.)

Regigigas ~ An temple opens only for those who are chosen. But to even see this pokemon in battle you need its compatriots.

Heatran ~ A mountain on an island of fierce trainers and pokemon.

There's also one other Pokemon that you can't see until Nintendo makes it available, but someone else is bound to mention it. Hint: The elemental plates you've been finding are really his.

---
And as an aside... the plural of pokemon is pokemon not pokemons. Same goes for each species too.

[Blademaster]

Darkrai, Shaymin, and Arceus are Event Pokemon, like Mew and Deoxys.

Heatran is in Stark Mountain on Route 227 - go through the whole mountain with Buck, and he'll take the Item at the end. After that, go to the Survival Area or whatever is west of the mountain, and talk to Buck in the house next to the Pokemon Center. Then go back to Stark Mountain; a LV70 Heatran is at the end.

Regigigas is in Snowpoint Temple. Get the other 3 Regis in your party and go there.

Cresselia is on Fullmoon Island. Talk to the Canalave City sailor and he'll take you there. Cresselia runs like Mesprit did, though, so don't panic if it flees right away.