I WANT SHAYMIN
It is no longer possible to get to the Flower Paradise (without the an Action Replay), but it dosent mean there wont be an event for it later on. if you want to find out about upcoming Pokemon events you could keep watch here
i think i know another way to catch shaymin in diamond... You still need AR but not the actual 'oak's letter' but just the cheat where you can walk anywhere... i dont remember where it is exactly but i think its either right or up of the pokemon league(somewhere in the sea) or at the sea right of snowpoint city. its been along time since i got shaymin so if i got a detail wrong sorry but im sure i got it this way.