Cabela's and Bass Pro have them as well as just about every sporting goods and feed store around me. You can also try searching the net. The ones I have been using are the small 4.5 ah batteries but there are bigger ones availible but dont fit in the housing. They run $10-$12 each and I even have a spare that I keep charged just in case.

I have purchased solar chargers for mine and they are working good, they have been keeping the batteries charged at 96% or better and they are taking 30-40 picture a day.

I am sold on the solar chargers and the 512 and 1 gig SD cards, even at high resolution you dont have to worry about not being able to take pics due to a dead battery or full memory.