I don't have a link, but I will paraphrase what I've read and tell you the names of some of the people who have denounced him :
Susan Harjo, President of the Morningstar Institute, has called him a liar. She said in 15yrs of interviews and investigations, he hasn't been able to come up with a single Indian ancestor.
And Vernon Bellecourt, an American Indian Movement activist in Minneapolis, calls him a fraud.
Churchill claims to be a member of the Keetoowah Cherokee, but that tribe has issued "associate membership" cards in the past, and even designated Clinton as a member.
Heres a link to some stuff on him. http://www.americanindianmovement.org/txaim/bellecourts.html
I knew VB when I lived up there - not well, but enough that I'd take his word over WC's.
Clinton has been known to claim Cherokee ancestry. That is about as reliable as most things he says.