It was my intention to go to an Optima battery when the time
came, but someone on this list correctly informed me it
wouldn't fit (there is only one size Optima).  Size specs at

http://www.optimabatteries.com/ (click "products")  Your
available space may be different than mine.

If in your area, available at http://www.batteriesplus.com/
Off-road shops probably too.

And, FWIW, type is "sealed lead acid" and not gel.