mtnbiker72 is correct. Shimano Factory MTB wheels do not use a "traditional" spoke/nipple arrangement. The spokes for their wheels are highly proprietary and only available from Shimano. The spokes are straight, have a pre-installed and permanent seat installed at the rim end of the spoke. The hub end of the spoke is threaded and is inserted into the hub and is secured with a threaded spoke plug.
So your pretty much stuck with whatever the wheel comes with.
Good Dirt