Try this site. Not sure where you're flying from or to, but it seems ot have some good deals.
Also try
Hotwire.
As Joe said, the seasons are reversed, but the typical US vacationer does travel a bit during Brazil's winter (our summer)...especially if kids are involved, so that's not a guarantee things will be cheaper. Fall/Spring might be a good bet because it's not Brazil's peak season weather-wise, but it's also not America's peak season, travel-wise. I too would definitely stay away from December if you're looking for a good deal.