Johnzaloog;10428 said:
It's a glitch you could do in the early Japanese releases of Diamond/Pearl where you could surf through a door into a "void" (the screen is entirely dark) and walk for a certain number of foot steps, go underground, resurface, and you'd appear in the part of the map where Darkrai/Shaymin (and other places) are located.
But the downside is that if you mess up, you could severely screw up your game, and you can't get to Arceus.
There are a lot of video tutorials on youtube if you want to take a look.
Garmmon;10224 said:
Or if you attended the events X3
Don't ask how I got the items. XD
Ahem, those haven't been released at all

.
Darkrai's event was a direct transfer of it, and there hasn't been a Shaymin event.
Obviously hacking is involved, not to point fingers or anything, but that's the only way you can obtain the items for the pokemon.