Johnson Controls, Inc. is one of 3 companies that actually build some or all of the batteries they offer in the USA; certain sizes come from Mexico, Saudi Arabia, or other parts of the globe (JCI marks them with a small sticker to identify country of origin outside America). Exide is another and the Duracell people may be the third. JCI makes them for Sears under the Die Hard label and for NAPA, and for Other companies that market them with their own name on them.
I have found that the best source for the 6 volt is Tractor Supply, Incorporated. While they are not that prevelent where Barbara & I live, TSI batteries are sold as they are farm oriented and various type of farm equipment still running after many years have this voltage.
Batteries+Bulbs stores in many areas have at least a couple on the shelf or can get product within 24 hours.
Be sure that you check the date code on the battery to see if it is "fresh" I have seen a 6 volt in one store recently (August 2018) with a date code of June 2017 for manufacture. As some of these 6 volt types only come with a 1-year warranty, this would be no warranty even before you got it out of the store.