Where can I find the original scientific articles on Heat index (published by either Larry P. Rothfusz or Steadman)?
After having a look at the references of a few papers about heat index (like this one). It looks like the original publications you are looking for are


The Heat Index "Equation" (or, More Than You Ever Wanted to Know About Heat Index) by Lans P. Rothfusz available here, and
The Assessment of Sultriness. Part I: A Temperature-Humidity Index Based on Human Physiology and Clothing Science, by R. G. Steadman available here.


As a general suggestion, looking at the references in related papers is always a great way to find the original research articles on any topic.