* Goldenrod Department Store: This is the most common way to obtain Dusk Balls. You can buy them from the Department Store in Goldenrod City, but only after you have defeated the Elite Four and become the champion.
* Pokéathlon Dome: After you have completed the Pokéathlon Dome, you will receive a Dusk Ball as a reward. This is a much earlier method to get one, but it does require some effort to complete the dome.
Let me know if you have any more questions!