Free: Contests & Raffles.
edit: One other trick I do: I have a 40 deg.ultralight bag and a 20 deg. ultralight bag. The 20 deg. bag is my usual go to. During the winter time I just take both and flop the the 40 deg bag over the top of my 20 deg bag. It's a lot cheaper combo them having a summer and winter bag. It's also not that much of a weight difference in the end.