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If I remember correctly that is one of those rectangular batteries with the metal cover over it. I don't know if Walmart can get it but they have some of the best batteries around and their prices are good. Interstate might have one. It is not a common item.
For those who want an original-looking battery with modern high-output innards, a factory in Germany will copy your original tar-top. It won't be cheap, though. I don't know if they'll copy the caps, if necessary. You could probably clone those in a 3-D printer, just in case: