When you say it won't start, I am assuming you mean the starter is not operating when you twist the key. If it happened suddenly, like overnight, clean the terminals and charge it up. Sometimes you can just loosend the terminals and twist them to improve contact, then go to the mechanic on Fremont and have him clean them properly and charge it up. Corroded terminals can reduce the amount of charge that gets to the battery as well as the amount of current that can get to the starter.
There is a small battery place across the street on Benito Juarez. Priced a new Kirkland (Costco) battery there $60. The Bodega was twice that. The have both new and reconditioned batteries. Friends have bought the reconditioned with good success.