Would this be a black Ghost knife you're talking about?
http://www.youtube.com/watch?v=JljQSDUvboc.
If so you can pick these up from your local fish store. Don't really see them that often on TM.
From the fish store expect about $40 - $50 for a baby.
They eat neons, cardinals and anything that will fit in their mouth when they're older.
main food are flakes, those vege wafers that you feed to loaches and blood worms aka red mosquito larvae.
Else you might be talking about a fire eel. They're pretty cool and on my big plan of things to get.