Jumping on the band wagon here, Sears, Costco, Price Club all sell the batteries you're after. One piece of advice would be to upgrade the pounds thrust of that motor. I use a larger one than the one you're planning on using for my Pram and one man pontoon.

If this is your only source of propulsion other than rowing, I'd be rethinking the motor size. If it gets windy were you are and there are two of you onboard, I'd super size the motor and go with a group 27 battery.

I'd rather have more power at my disposal and not need to use it all of the time, than not have enough power and find out I really need it some time.

Just an opinion...Dave