Check with your local distributor? I have seen the 1/6th sitting around at my distributor - they are mostly the local micros, etc. but I'm sure you can get others as well.
Another option is asking a microbrewery to fill a corny for you. Most will oblige with some advance notice and for the right price. It's no different than filling a few growlers for them...
Roaring Bull Brewing Co.
Currently Consuming (HB): Apfelwein on Tap Troegs Hopback on Tap; Craft Bottles
Fermenting/Conditioning: Up Next: Hop Trio American Wheat, Lake Walk Pale Ale
In Planning Stage: Farmhouse Saison and Something Oaked.