For this project we used an assortment of continuous operation lasers from Laserglow.com
's Brightline Series. The lasers ranged in power outputs of 25 mW to 40 mW, but you can use lowered powered units, but it won't be as awesome.
We removed the mounting brackets and prepared them with matching power supplies to be placed on the tree. If you don't have access to continuous operation lasers you can use a regular laser pointers by taping the button so it stays on. Your battery will die eventually, but you'll have a temporary solution to create your own display. Green laser pointers can be purchased from both Laserglow.com