... (Like a letter or decree) where she expresses this (or where a pope or someone discusses it)?
Talking about how she thought the issue over when some natives arrived in Spain, and ended up sending them back home, opposing slavery.
But looking for the original sources for our understanding of this.
You are looking for the Royal Provision of June the 20th of the year 1500. Let me translate it for you from the Colección de Documentos Inéditos relativos al descubrimiento, conquista, y colonización de las posesiones españolas en América y Oceanía, 2nd series, volume XXXVIII, pages 439-40:
The King and the Queen.
Pedro de Torres, contino of Our House: You already know how you have under confiscation and deposit, by Our command, some of the Indians that were brought from the Indies and sold in this city its archbishopric, and other parts of Andalusia by command of Our Admiral of the Indies who, know, we command you to set free; and we have ordered Comendador Frey Francisco de Bobadilla to take them under his power to the said Indies and do as we have commanded. Henceforth, We command you that, as soon as you see this Our royal charter, give him this Indians that you have thus under your power, without missing a single one and inventoried by a notary public, and document how he receives them from you; and with that and with this Our royal charter we command that they shall not be demanded from you, and you shall not do otherwise. In Seville, 20 days of June of the year 1500.
By the King and Queen's command, Miguel de Almazán.
And then there is the codicille supplementary to queen Isabel's testament, where she is abundantly explicit in clause XI:
I also command that, inasmuch as the Pope granted us the islands and firm lands of the Ocean Sea, discovered and to be discovered, and as it was my intention to induce and bring the peoples that populate them to the Catholic faith, and to send to those islands and firm land prelates and clergy and wise people to instruct the inhabitants in the Catholic faith and teach them good customs. I also pray very affectuously to my lord the King, and charge my daughter the princess and her husband the prince with so doing and accomplishing, and that this be their main goal and put much diligence in it, and that they shall not consent nor leave openings for the Indians, neighbours, and inhabitants of the Indies won and to be won to be torted in their persons or goods. Furthermore, that they sall be treated and fairly, and if they have received any torts, shall them be remedied, and be it provided that no one shall trespass on the apostolic letters and what in them was established and contained.