Original AC adapters for these systems are very difficult to find, and when you do find them they can be quite expensive. Many universal DC adapters do not come with a plug that fits the this unusually sized DC jack.
Equivalent of Radio Shack Adaptaplug "N", p/n 273-1717
Mattel Intellivision II
NEC TurboGrafx-16 CD Dock
Sharp Twin Famicom