For two years I have purchased the landscape fabric at Sam's. It is a commercial grade and is about twice as thick as anything I have purchased at Home Depot or Lowe's. That is important because I walk up and down the middles all year. It has held up beautifully. If you are gardening on a smaller scale or in a raised bed, the thinner fabric might suffice. We also have purple nutsedge, which will punch through most anything, but it has yet punch through the Preen.
Here's a link to the exact product I've been purchasing:
Preen landscape fabric at Sam's