While Maerzen or Oktoberfest beers usually arrive into the U.S. from Germany in better shape than many imported styles, there are some excellent "O'fest" style beers brewed in the U.S. as well.
The Samuel Adams Oktoberfest is a fine and widely available choice. Regional breweries with worth-watching-for-if-you're-in-the-area Oktoberfests that I've tasted recently would include Dominion Oktoberfest from Virginia's Old Dominion Brewing Co. (available in the mid-Atlantic states) and Gemuetlichkeit Oktoberfest from Wisconsin's Tyranena Brewing Co. (available in Wisconsin and parts of Illinois). The branches of the Gordon Biersch brewpub chain (they're in more than a dozen states) use the same recipe chain-wide to make excellent Oktoberfests, and it's hard to find a fresher beer than one made in the next room.
I'm sure that there are other local and regional O'fest suggestions that can be made, and I'm looking forward to reading them on this thread so I'll have some ideas of what beers to look for.