If your looking for a basic Group 27 battery, you can buy them from anyone from Advance to Walmart, nothing special about them. Some have different cranking amps and warranties, but a Group 27 battery just identifies the battery for case size and battery post location.
If your looking for the correct style reproduction battery, various vendors have them, but as mentioned above, always call and verify actual availability. Antique Auto Battery was the supplier for years, but is now operating under new ownership as Jim's Auto Batteries. They basically build the batteries in house and it is sometimes a three to four weeks turn around for them. The other choices is Battery Centeral Mall and Turbo Start. I know that Turbo Start has been working on the release of the batteries for over a year now and it seems that it is still in the works.
A year ago at this time, it was almost impossible to find a new reproduction battery and a three to four month wait was not uncommon. Thankfully, they are back available, although it can still be a couple of weeks to get one.