Dream trees are located at the Woodcutting Zone (::wcz), Donator Zone (::dz) and within Wilderness Resource Area. You require level 90 Woodcutting to cut these trees, and it will provide you with Dream Logs. Whilst you cut these trees, you sometimes fall asleep, prompting a "zzz" message in chat, and interrupting your action.
Locations
Woodcutting Zone - Head to the Woodcutting Zone, by using ::wcz. Then, run East and you should see 4 Dream Trees.
Wilderness Resource Area - WARNING* - This is located in the Wilderness, and therefore is NOT safe.
Head to the teleports by clicking the MAP, then go to Player Killing, Page 2. Then click mage bank and teleport. Use the lever to enter the Wilderness* and run East. Head passed the Hellhounds, and you should see a gated off area, with 3 Dream Trees. It requires 10K Gold to Enter, and you can use Giles to note any items for 2500gp. You will also gain a base increase of 1.5x exp for being in the wilderness, which makes it a favourable but risky place to train.
Donator Zone - If you have the donator rank, you can use ::dz to go to the Donator Zone. The benefits of using this area, is the trees are infinite, and therefore the trees do not run out.