Verified answer

Migrating from FireRed/LeafGreen

or

By going to an event

There are only 2 ways to get suicune other than trading or hacking. 1. Complete pokemon colleseum and purify venus's shadow suicune, trade to the gameboy and migrate to DS. 2.Complete firered or leafgreen with a charizard. Get to the island and capture suicune and migrate.

Basically you have to migrate it from the gameboy after capturing one in colleseum or LG/FR

There is only two ways:

Get Suicune from FireRed or LeafGreen by choosing Charmander as the starter pokemon. Once you get the national dex, it goes off in the kanto grass. Find it in the wild and capture it and transfer it to your game.

If you have Suicune in your pokedex and if you have wireless internet that you can connect your ds to, you can get the Suicune off of the GTS (Global Trade Station) in Jubilife City.

Migrate from Fire Red and Leaf Green.

you have to get it on the gameboy advance game and then trade it over to diamond with pal park

the only way to get one is from migrating from fr/lg

BUT

you can look on the hubs at friendcodes.com thats where i got mine

you can only trade it via wireless from pokemon silver or crystal version