• fabric stores
  • really... it gets used a lot in interesting projects
• sometimes in hardware stores, home-improvement stores, and garden centers
  • Ace Hardware, supposedly
  • some people even report it being sold sometimes in Walmart near the lawn chairs
  • others report Walmart/Kmart having it in the arts&crafts area
  • sometimes it's sold for lawn chairs
• Army surplus stores
• places with climbing supplies (though climbing webbing is generally pretty serious stuff)
  • REI
    • DING DING. We have a winner. Yes, it IS pretty serious stuff. Surprisingly, it isn't seriously expensive stuff. In fact, sometimes the climbing-grade webbing is cheaper than non-climbing grade stuff. But they also have less-than-climbing-grade webbing at REI too. Good place!
• the Wikipedia article mentions that the transportation industry is the biggest user of webbing, mainly for tie-down straps (though, again, usually I don't require anything NEARLY this beefy)

Of course, there are tons of online places you can buy it. [1]