I think you can get those heat packs at Daiso/other 100 yen shops.

When I went to Japan in January 2014, I saw tons of (fashionable) girls wearing Uggs. Not sure if they're still in style in Japan, but I had brought mine to wear on the plane (I always get cold) and ended up wearing them around Tokyo because it was cold, but not wet. Layers are good. I did end up wearing long underwear pants under jeans, but like I said, I'm always cold.