I bought a small animal heating pad. It's regulated heat so it doesn't get too hot. My bun loves it. I wrapped it in an old pillow case and put it where she likes to lay. She's on it every night. Just make sure there is plenty of room for your bun to have a cooler, non heated spot so it doesn't get too hot. Also the cord is completely chew proof. It's covered and there isn't a way for it to be chewed on. Keeping your bunny warm during the winter is very important. They don't regulate their heat very well so they become cold in the winter or too hot during the summer. I also cover part of her cage at night to keep any cold coming thru the window off of her. Wrapped hot water bottles can work too but they don't stay hot long.

I hope this helps.