I hate to break it to you, but you can not buy berries. If you run out, the guy in the Pokemon contest buildings, by the blender, if you talk to him directly, he will give you a berry only if you don't have any on you.
*note: he only does this once a day
No, you can't buy berries in Pokémon Diamond.
http://www.psypokes.com/rs/berrystats.php
You have to wait for it to grow.
Mulch
The leichi berry raises the attack stat if held by a Pokemon (that what it says in heartgold but I am not sure if it's the same in emerald)
No, you can't buy berries in Pokémon Diamond.
http://www.psypokes.com/rs/berrystats.php
In emerald you get them in the contest hall by making them with berries.
Mulch
You have to wait for it to grow.
you can buy Pokemon emerald, ruby, sapphire, firered and leafgreen at gamestop
Feed them good pokeblocks from durin berries.
The leichi berry raises the attack stat if held by a Pokemon (that what it says in heartgold but I am not sure if it's the same in emerald)
you buy it
you buy it
You can buy it at Gamestop.
You cannot buy berries, but you can find them on trees, on wild Pokemon, or from another game.