Thanks to Ladybug!
First of all you need to equip the myst bag, which you will find in Sapphire Caves, then you need to go:
1) In Green Grass on the first screen you get coming from the world map area- it's on the top left side
2) In Scarlet River on a bush near a locked chest - you find it to the right of the main path shortly after the place where the fight with the octopus was held
3) In the Merchant's Shop Secret Room
4) In Rocky Path on a rock in the small lake with the boat in it
5), 6) and 7) In the secret church the ghost boat driver takes us sometimes
8 ) In the Rogue territory towards the right - on top of a tree 1 screen left from place where Rogue was heard
9) In lower section of Rocky Path just as you climb down
10) In Sapphire Cave in between two blue lights 1 screen left of where Red flame was
11) In Ancient Temple to the Northeast in a room with two golden statues
Quest Complete!
Even though only 11 are necessary for a complete quest, more have been found, namely
12) in the shrine north of Green Grass
