I'd be willing to bet the 90 day and 5 year batteries are substantially different.
I bought an Interstate MTP-65 Mega-Tron Plus 2 days ago from Northern Tool for $130. Interstate website shows this battery lists for $173 with a suggested list of $151. Not sure what other dealers sell them for, but I thought that was pretty good. 6 yr performance (warranty?), 5 yr replacement warranty.
I didn't look for 6V batteries, but they had a pretty good type variety. Tractor Supply might be a good place to check also.
I like the blem idea . . . . . if only I'd known.
Best,
- bob