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.
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The best ball to catch Giratina is the Master Ball but the next best and effective options would be the Dusk Ball or the Ultra Ball.
use a ghost type Pokemon with scary face so it cant run away and then use a dusk ball or a net ball and you will get it don't fly or it will run away
Go underground and dig.
a dusk ball when he is on yellow but use a 23 and under Pokemon and you can find the dusk ball at the inside of the Pokemon legue at the right counter
That was a Dark Ball.