level it up with a rare candy then it evolves
sandshrew can eventually learn it at level 44
You'll need to evolve it from Sealeo at level 44.
route 44
In the Platinum it is 44, but I am not sure about Heartgold and Soulsilver.
Route 44.
go to route 44
They can be found wild on Route 44.
at the end of route 44
You can catch him at the Route 44 Safari Zone.
Put 24 Peak objects in the Rocky Beach Zone of the Safari Zone and Aron will appear (at a very high level- 44-45!)
you can remonaid on route 44 in the johto region
In Pokémon SoulSilver, you can find a Level 44 and a Level 45 Aron in the Rocky Beach area of the Safari Zone however you will need to place the exact amount of 24 Peak Objects in the rocky beach area before the Steel and Rock-type Pokémon; Aron will appear in the area.