Can anyone find me a legitimate article?

Question

that says that the US has plenary power over Indians on Reservations, they are not using it.

Answer 1

Though Wikipedia is never a perfect source, their listing on plenary power mentions the case, United States v. Kegma (118 US 375).  The case decision can be viewed at a number of legal websites.

A further search for the phrase "plenary power" restricted to the .gov domain brings up a page from the US department of Justice - division of tribal justice and safety.  Included is the statement:

"Though generalizations are difficult, a few basic principles provide important guidance in the field of Indian affairs: 1) the Constitution vests Congress with plenary power over Indian affairs; 2) Indian tribes retain important sovereign powers over their members and their territory, subject to the plenary power of Congress; and 3) the United States has a trust responsibility to Indian tribes that guides and limits the Federal Government in dealings with Indian tribes. Thus, federal and tribal law generally have primacy over Indian affairs in Indian country, except where Congress has provided otherwise."

As far as I can tell, the US government does use the power in the administration of justice (investigation of crimes which occur on tribal lands), so I'm not sure how much that helps in that regard.