Mist Wood can be found in grassy areas of Wizard City. In particular go to Unicorn Way, Cyclops Lane, Triton Avenue, or Firecat Alley and look in the park areas as well as around any towers.
If your not having any luck, try switching servers. Often times one server will be empty of reagents where another may be filled with them. This is usually due to some servers being highly trafficked and thus regeants are snatched up pretty quickly.
If all else fails, you can always buy some from the Bazaar. Just go to the Regants section, there's usually plenty there.