Can I substitute a stainless steel pot for the traditional iron dutch oven?
Cast iron is ideal, but any pot that can take the heat and has a tight lid will work. Like @talon8 said in his comment, it doesn't even have to be metal.

This article from Around the World in 80 Bakes specifically uses terracotta for sourdough, not cast iron.

Just as an FYI, this related question deals with preheating (for no-knead bread, not sourdough), and the differing answers are interesting. To me it just goes to show that bread-making doesn't always have to follow super-strict rules. Preheat the Dutch oven (and the oven itself) for No-Knead Bread? (experiment results)