Buy a new one.
You can't catch it in Pokemon Platinum but you can get it in Pokemon Pearl in Eterna Forest at night or in the Lost Tower.
lost tower at night
go to your lost tower
rt.209
no
you cant stupid.
Beat the Pokemon league again and go back to spear pillar and you will see dialga again.
it for the powerful Pokemon home "spritomb!"
lost tower 5th floor
You can get gastly at the Lost Tower or in the Eterna Forest
You can catch Duskull in Pokemon Platinum by going to the Lost Tower that is near Solaceon Town however you can only find it at night.
MurKrow cannot be found in Pokemon Platinum. To get MurKrow you need to trade with a copy of Pokemon Diamond. In Pokemon Diamond MurKrow appears in Eterna Forest and the Lost tower at nighttime.