Use the pokéradar in the grass on top of Mt. Coronet. It may take a couple trys.
You'll need to get it as an Egg from Loudred or Exploud.
On your way to the Spear Pillar the summit/snow use your pokeradar to find Loudred and evovle it at level 40
No where unless you cheat or get loudred to level 40
you cant.But you can catch loudred in the mt.coronet than you can make an egg also if you dont want you can migrate it from sapphire ruby and emerald
You can't catch whismur in Pokemon pearl or diamond but but you can catch its evolved form loudred with the pokeradar on the top of mt coronet.
It can be found in Victory Road.
Go to either Mt. Coronet 3F or Mt. Coronet Summit (First Half) and use the Poke Radar to search for a Loudred.
Breed a Loudred and hatch the Egg
Get the national dex and transfer from ruby, sapphire, or emerald or in Mt. Coronet on the snowy area by using the Poke Radar
Mt.cornet use a poke radar (it is rare)
You can find Loudred in Pokémon Sapphire in Victory Road additionally you can find wild Whismur in Route 106, in the Rusturf Tunnell and in the Victory Road and then you can raise up Whismur until it reaches the level of Level 20 at which point it will evolve into Loudred and another option is to trade with another Pokémon Trainer locally in order to receive Loudred into Pokémon Sapphire.
Loudred evolves at lv.40