Well, ebay, or find a local store that refurbishes and sells them bad boys. You'd be surprise how many places you can find them.
Are you dead set on getting one of the originals or do you just want to be able to run the cartridges?
If you just want to run the games, I would definitely buy an FC Twin
. Review of FC Twin
. You can find those in local game/comic shops a lot of the time. Call around and ask what they have in the way of 16-bit console emulators.
Link to Seller
. There's also sellers listed on Amazon for it.