Migrate one from ruby, sapphire or emerald then capture it in the pal park.
Not in Platinum but in HG/SS you can
Unless you cheat it is impossible.
You can't catch rayquaza in platinum unless you migrate it from ruby, sapphire or emerald.
You catch Raquaza in the Sky Pillar on the island near Pacifilog City.
Catch raquaza
I only know two ways to catch raquaza in only platinum first you can make your way up Stark Mountain and right before you enter the mtn. run in the big patch of grass while using your pokeradar and the other way is if you know where Prof. Oak is you have to (have Lugia, Moltres, Zapdos, and Articuno for this to work) talk to him and he says "Ahhh (whatever your name is) I remember seeing Groudon, Raquaza, and Kyogre have entered the region."
Wild Rayquaza can only be found in heartgold/soulsilver. Either catch it in one of those games and trade it to Platinum or get one from the Global Trade station (Jubilife city)
you have to migrate it from emerald,ruby or sapphire.
no you cant get rayquaza
Sky Pillar
catch raquaza
depending on the game there r certain Pokemon u cannot catch like say on emerald u cant catch palkia or dialga so on diamond u cant catch raquaza sorry