Timer balls are hard to find in Pokémon Brilliant Diamond and Brilliant Pearl, especially at the beginning of the game, but by the time you reach the second half of the game, they are much easier to obtain.
You'll find these special Pokéballs in sleepy Celestic Town, where an ancient Sinnoh shrine resides. When you arrive, you will see that there is no Pokémart. Instead, there is an independent operation in the upper left of the city. It's like visiting one of those little neighborhoods in the middle of nowhere in the US or Canada with no big brands to talk about.
Since it's off the beaten path, you'll need the hidden anti-fog move, as well as Cynthia's potion to take out the Psyduck block to reach Route 210.
The shop is located near a green-haired woman and a small pond. It is exactly in the upper left corner of the community. There is no signage, nothing to indicate that there is actually a shop there. Enter and you will see two elderly cashiers. Talk to the man on the left. He will offer you Dusk Ball, Quick Ball and Timer Ball for 1.000 Pokédollars each.
When you get a Timer Ball, it is best to use them when the battle has lasted for a long period of time because their effectiveness increases as each turn passes. If you've used a Master Ball and cleaned up Ultra Balls, the Timer Ball might be your best option if you've been keeping a Legendary Pokémon like Lugia at bay.