Any of the grass/forest map sections if you're on land. There's actually a location guide for all obtainable Pokemon under the "Maps" tab, for future reference. You also have the choice of buying any of the starter Pokemon from the Pokemart.
Chat with our AI personalities
just battle with a Pokemon
you will find beldum in the cave
In the grass area
none
Go to a map, walk around until you find a Pokemon, and catch it.