I am not sure. If the word "denialist" is new (which it is), you are unlikely to find a source calling an organization 100 or 150 years ago "denialist". You might find all the characteristics of what denialism is of course. 

It depends on how you define denialism. If we define it very narrowly, we will only end up with 1 or 2 topics in the category of denialism, which clearly is wrong given the similarities of human behavior in those one or two areas with those in other subjects. 

I believe we should do what dictionaries do when they define a word: we look at prominent sources like the New York Times, the New York Review of Books, the London Times, the Economist, the Wall Street Journal, Academic  journals, scholarly writing, popular novels, etc to see how the word is used. For a new word, this determines its meaning, until a family of dictionary editors codifies this set of meanings as a family of different definitions that are published in dictionaries. Since we are not yet at the dictionary publication stage, but at the earlier pre- dictionary publication stage, we have to do more of the hard work ourselves. That is all. 

It is clearly a common word, in common usage, at least in some subject areas. One rarely hears the word "AIDS denier" although that is essentially what an "AIDS denialist" is (parenthetically noting that in fact an AIDS denialist is really an "HIV denialist", properly speaking). And I suspect the phrase "Holocaust denier" will fade from usage now that the new word "denialist" seems to be gaining popularity. 

If one maintains that there are two separate groups, denialists and deniers, then why are there no AIDS deniers? Why no HIV deniers? Only denialists? Seems a bit strange, given that human behavior is not much different in all these cases; only the topic changes. The denying stays the same. 

If putting someone in a denialism category constitutes libel, then a very large number of publications worldwide have put themselves in legal jeopardy by publishing articles on AIDS denialists and other kinds of denialists. Given that these publications have full time legal staff and large legal budgets, and the people making these claims on Wikipedia are not lawyers or legal experts, this claim seems dubious at best. 

If we accept the gratuitous personal assertions of some that denialism is really a political movement that involves personal gain, then creationists, creation scientists and intelligent design promoters fall in this category. If "denialism" is more like denial of the evidence, then this is less obviously true, since there is a dispute about the evidence in these cases. 

If we define "denialism" as some here claim, to only mean those that know they are lying about the truth, then where does that leave the most common usage of the word in the phrase "AIDS denialism" ? It obviously cannot mean "lying knowingly" in this case, or probably in all cases. 

I suggest that if one requires a publication to directly make the claim that "X is a Y", to put some article in a category, then we would depopulate most of the categories on Wikipedia. If Y has characteristics {Y1, Y2, Y3, Y4...Yn} from dictionary definitions or from common usage, and this common usage is well-understood and evident from publication in WP:V and WP:RS sources as defined above, and if X has most if not all of the characteristics {Y1, Y2, Y3...Yn}, then surely X can be put in the category Y. 

For example, suppose we define a new type of urban area called a "Townlet", being an area with a common administration within a radius of 5 kilometers and a permanent human population of 5000-10000 people. Then one surely does not need a publication to state that "Filllsberg is a Townlet" to put Filllsberg, meeting all the necessary criteria, in the category "Townlet". 
 
I also vehemently deny the assertion that "denialist" is a singularly negative category and so should not be used. If so, we would get rid of a large number of categories, such as "pseudoscience". 

Also, as I have stated repeatedly, "transhumanists" or "secular humanists" or "aetheists" might be viewed as denialists by some. And might feel perfectly proud to proclaim themselves as denialists of some overwhelming belief or putative evidence. 

For these reasons, I think there is nothing wrong with having a category called "denialist", or "denialism". The membership of this category might be controversial, depending on what we take as the definition of the word. Any particular subject might or might not be put in that category, given community consensus on the word's definition, and on the individual case by case analysis of any particular topic, subject or article.