I have never needed one myself, but I saved this link because it was highly recommended on another car forum I used to frequent.
http://www.jorgenauto.com/index.html
They ship you a remanufactured unit with a return shipping label and when you ship your old one back to them in that same box they refund your core charge. At the time, I looked up the cost to get one locally and even with the core charge and shipping it was cheaper than what I could find anywhere else without the core charge.