do you speak Spanish? lot of resources out there in Spanish, if you do I can point you towards few of them.

Colon wrote more than 4 letters as you can imagine his correspondence was rather large, that is the link I provided. Yes, there are originals and/or copies of many documents from other explorers like Hernan Cortes, Pizarro, Elcano or Malaspina. You can obtain free digital copies from the Biblioteca Nacional, Cervantes virtual Library and the Archivo de is one of the Indias (Archive of the Indies). That is one of the reasons why I initially asked if you spoke some Spanish as you will need it to surf those databases. Some have English pages but the documents and their titles will be indexed in Spanish. Hope you enjoy what you got so far.

http://www.bne.es/es/Inicio/index.html

http://www.cervantesvirtual.com

http://www.mcu.es/archivos/CE/ExpoVisitVirtual/visitas/index.html

Right, Colon (as in the original) first letter is lost to us, we know it through Bartolome de las Casas who wrote about it. http://www.unm.edu/~mcknight/colon.htm a simple internet search will give you both Spanish and translated versions.

On to Cabeza de Vaca as far as I know the original is lost too, it came to us via Gonzalo Fernández de Oviedo. You can also get facsimiles of old editions from the the Spanish Digital Archive of the National Library and translated copies from Livribox in the public domain.