So a disclaimer: That Decoration present in mint condition with the certificate was purchased on eBay for a small fortune. My second 'played' condition decoration present came in a lot on buyee for a few hundred (was a very good deal).
You will want to look on wikipedia for the japanese names of Pokemon Battle-e/e-Reader/Pokemon Colosseum/Decoration Present/etc and then search in japanese. Save several of these searches.
Another disclaimer: I found out about Buyee and got most of what I wanted BEFORE it was cool...the cat's out of the bag now, so competition is more fierce.