Ever see a cold bear, Or Cat?. Fur is the best insulation around, next to Feathers & blubber. Hard part finding the fur coat to cut up?. Depends on type of fur, as well.
Try beaver, black bear, caribou, Otter, Foxes.. critters of the northern hemispheres.
Another option for finding more Down comforters, Visit a Dry cleaners, see if they have Old unclaimed down covers laying Or Large wool coats.. Pay the cleaning bill for it.. they might be happy to get money and unclaimed item out of the store. Its what I did.