This is obtainable before the end of stage 1-3, if you look around the first 3 levels religiously and break all the rocks containing items.
I found 2 coins in 1-1, 1 coin in 1-2 and 2 coins in 1-3.
If you reach the boss battle in 1-3, don't worry you can still go back and explore the rest of the level after defeating him. Just be careful not to walk too far to the left.
Trophy unlocks after collecting 5th coin. All coins have to be collected in one game play.