I going to visit United states and its famous geographic places, such as Grand Canyon.
I heard that there are some helicopter tours are available during visiting the Grand Canyon.  
Is it true?  
How much it cost?  
What firms are famous for its service?
What documents should I have to get this tour?
Any info is important.

![Grand Canyon image][1]

  [1]: http://i.stack.imgur.com/d5zmQ.jpg