I usually buy them on Amazon. Their availability schedule is kind of wonky, but they are usually up within 12 hours after air time. As mentioned in the previous comments, there are other options as well.
It will be up on Amazon to purchase sometime soon.
http://www.amazon.com/Gravity-Falls-Season-2/dp/B00MEU8LR8
Though, if you don't want to support the show, there are always illegal websites where you can torrent it, which I do not recommend.