Locating primary sources for SPAN 308 is not often easy or obvious. There are a couple of tips for future research projects.

To begin, figure out the controlled vocabulary or heading of the person you are researching. This is the language that libraries and research databases and resources use to pull information together about a particular person. Like a hashtag. So we might refer to Queen Juana or Joanna of Castile but for the purposes of research, she is known as:
Juana, la Loca, Queen of Castile, 1479-1555

To locate primary sources connected to her, you might add the word “Sources” or “early works” or “Correspondence” (for her letters) or “documents” to her name.

Next look at any Wikipedia or biographical article on the person. Read closely the external notes or references at the end of the piece. Are there books or resources mentioned there that use words that might look like collections of official documents, correspondence, archives of papers?  See if you can locate them on the Internet. The Internet Archives and HathiTrust store a great number of digitized resources that are very helpful. 

For Queen Juana la Loca, I found an article by Bethany Aram looking at her signatures.  Since signatures are connected to primary source documents, this might be helpful. Especially the notes where indications of the documents referenced in the article will be listed. That is where I found the collections of documents from the history of Spain listed below. I also mined the external notes and references of the Wikipedia article on her.

Calendar of letters, despatches, and state papers relating to the negotiations between England and Spain preserved in the archives at Simancas and elsewhere
https://archive.org/details/bub_gb_9q8MAQAAIAAJ/page/47/mode/2up

It’s a scan held in the Internet Archive. Look at pages 47 and onward for letters (including the English translation) to and from Queen Juana. It looks like you will need to go through the pages and look for letters that are from the Queen.

This website also might be useful. These are state papers of Spain. You can look at the papers for the years Queen Juana lived and see what documents she may have written or signed.
https://www.british-history.ac.uk/series/calendar-state-papers-spain

This collection of documents also has letters and correspondence from a variety of Spanish rulers including Juana, housed at the Internet Archive. 
Coleccion de documentos ineditos para la historia de Espana, see volume 8. Especially page 291-293. You can use the “Search” feature on the left side of the screen to Search inside the book. Look for “Juana” or “Dona Juana” to find places where she is mentioned. Then look and see if this is connected to a letter written by her.