6 trees in the Resource dungeon found in the Varrock Sewers by the Moss giants.
3 trees (4 when using the Farming patch) scattered within the Tree Gnome Stronghold In a straight line west to east. The centre tree is just a little south-west from the bank. The west tree is on the other side of the river next to the Terrorbird pen (south-west corner). (Note: There is a south gate on the pen.) The east tree is just outside the Agility training area (south-east corner).
4 trees at the Sorcerer's Tower (often very crowded)
3 trees east of the Ranging Guild (they are also just northwest of the Sorcerer's Tower, so players can go back and forth between the two locations)
2 trees surrounding the Mage Training Arena
3 trees in the south-east area of Isafdar, just south-west of Lletya (less popular due to high quest requirements)
1 tree in the Wilderness, which has a counterpart: the cursed magic tree that is found in the Spirit Realm (unpopular due to its distance from a bank)
1 tree south of the Gnome Stronghold agility course.
http://runescape.wikia.com/wiki/Magic_tree