Go to Pastoria City and there will be two people at the counter. Press A in front of the guy on the left and he sells dusk balls, quick balls, and i think net balls.
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You can buy them at the Pokemon mart at the Pokemon league. I buy my timer balls, heal balls, etc. at the Pokemon mart in canalave city
there isnt any dusk balls in fire red, they were introduced in generation iv
Go to the pokemart ( Velistone and the Pokemon League should have a supply of dusk balls, otherwise you may come across them by having a Pokemon with the ability "PICKUP" or finding items using the dowsing app on the Pokewatch.
you can get dusk balls at the elite four hospital along with a bunch more different ones that are really useful.
You can buy the dusk balls in Pokémon Diamond at various markets in the game. The dusk balls cost 1000.00 a piece.