http://www.detailedplay.com/
this place (and i am sure there are others) has plans..
and this place has info on Labyrinths
http://www.labyrinthsociety.org/
I enjoy and appreciate Labyrinths, and have offered to
paint one on the local playground asphalt...
I have several, portable ones painted on tarps,
planted in the garden, with paths to walk.
here is another site with ideas..
http://www.timberform.com/themed1.cfm