Secret Stoneplate or Enigma

They are obtained via optional missions. Look for ones that say something about Mystery Parts, or Secret Slab.

There is a very easy wondermail code #C3W P4&X 3Q86 552C &6K- 8PP5 you have you find the mystery part on the 4 floor of beach cave you take a client[hoothoot] but at the end you don't have to give it to him so it is like an escort

You can get the Secret Stoneplate or the Enigma Part as rewards for some missions. Or you could just use the Wondermail code that RekiEXE gave.

You can get them from wondermail and you get them fora mission for finding it on a floor.

There is also a wonder mail generator where you can generate your own wonder mail with any reward you want!
Http://www.upokecenter.com/games/dungeon2/guides/wondermail.php

just to let you know the item is no good untill you complete the main game

Here are wonder mails I found:
For Secret Slab:

@7PCTM3J#Q@Y
2T=8K2CYXX5K

For Enigma Part:

Y#4FHSY51P

Here's a code for secret slab
You need to beat a magikarp whit venesaur in team on beach floor 1:::
XTW& MP6Y 2F2X
CQ13 PH%C -HF8

The awesomeness of Lugia is unbearable but you need the enigma part or stone plate

So you get Lugia? Well anyway just follow the codes to get the parts then somthing should up about a dugeon go there and whatevers there