You have a few options. As said above, there is eBay.
Then you have Amazon. Check Amazon Warehouse deals (their used section). There is even a thread dedicated to it in the video game deals section.
Last but not least is the trading section of the forums here on CAG. Browse the topics, mabye a fellow member has one for sale or trade. I know there is a topic in one of the forums, I can't recall which, about OOT 3DS and one CAG sold his sealed copy for 75 to another member.
Then there is Craiglist....
Who knew this game would become such a hot commodity? Good luck my man and happy hunting.