It's sold by Fanatic Outdoors and costs $55 plus $8 shipping.  Here is the website:

  Fanatic Outdoors 

When I received it my first thought was that I couldn't believe I paid $55 for something so simple.  However, it really works well once you get the hang of it and I love how light and easy it is to carry.  I just clip it to a belt loop or D-ring when I'm on the move and it doesn't even feel like it's there.  I also have a Millennium M300 tree seat that is really comfortable, but it weighs 4 lbs and it's awkward to carry around all day if you're running and gunning.

Here's another video for it:

  Hammock Seat