Yeah, they're seasonal for sure. But all of those breweries offer bocks and most of them offer doppelbocks, so if you want malty German goodness outside the Autumn months, there are always options.
Today I'll go local, my three favorite beers are:
1) Oaktoberfest - Live Oak Brewing, Austin, Texas
2) Oktoberfest - Saint Arnold Brewing, Houston, Texas
3) Montecore Oktoberfest - Austin Beerworks, Austin, Texas