The green wolf eels are very common (actually a blenny) in the trade. These fish reach about a foot and are voracious hunters.
The giant atlantic/pacific wolf fish (also a blenny) is protected, in the US, and can't be kept in other than public aquaria. There are 5 species. You may be able to keep the oriental wolf fish but, I've never seen one even in public aquaria in the US. These fish can reach 60" and require pristine cool water with a regular diet of sea urchins.
Green morays (Gymnothorax funebris) are another commonly available species. Plan on dropping a couple hundred for this species. (along with providing a huge aquarium to fit these 8 foot+ eels.)
As far as conger eels, you need to define down which one(s) first. There are over 100 species that are known by the term 'conger' eel. And, a number of them reach over 6 feet.