We use
www.batterymart.com at work.
You can get a 6v or 12v sla batt from them for $9.95
As for charging, the best charger out there is prolly this one:
You can leave this one on your batteries between shows and it will NOT cook them. It charges em, then keeps them charged with a pulsing charge. I have one on my motorhome for our house batteries, and I used them for a number of other sla batteries that I have.
The initial price is high for the charger, (bout $100) but it is worth it because you will get 5 yrs + out of your batteries.