The Batteries we recommend for most our installs is Golf Cart 6v batteries from Costco or Sam's club.

There ~ rated value is 220ha 6v batteries 3 cell 70lbs each.  They are very nice batteries and most of them come with a threaded positive and negative lug to attach your connections too much easier than others.

Average cost is 75 dollars and these batteries are made by Johnson Controls out of California.

They are made from recycled lead and plastic which is a plus in my book when you have to use batteries which are one of the most toxic consumer products to go into landfills. 

Costco and Sam's offer two different models of there batteries.

Costco's is a translucent white plastic battery container with a red top as seen below
Sam's club has a all black version with black top.