If you have one, try Tractor Supply Center. They have a lot of different sizes, I got the exact fit battery for my '41 Buick, it's an Exide and they had it in stock, under $100 exchange.
Rural King carries two sizes of 6volt batteries. I am using the smaller one in my '55 but the larger one would have fit the battery case perfectly. Don't know if T-Bird battery is physically different shape though