Take the battery to CarQuest, Autozone or the like and have them run a diagnostic on it, it's free. They will tell you the batteries dead and they need to charge it first before they can test it, so come back in an hour. If you hadn't put any fuel stabilizer in the bike before you let it set, go home and drain the gas and refill with fresh. You can put the old gas in your car. A couple gallons of old fuel diluted in 20 gallons isn't going to do anything.
If they tell you your batteries good, well then they've just charged it for you. If they tell you it's bad, buy one from them. Just make sure it's the sealed, maintenance free type. Not the kind you have to add fluid too. As long as it's the same physical dimensions and same or more CCA's, it'll work. I have a Carquest powersports battery in my bike over 2 years and it was only like $36 IIRC, prorated 3 year warranty. Some people swear by Yuasa though.
As far as battery charger, anything that's labeled 'automatic' is good. Steer clear of 'manual' battery chargers.