I used organic heirloom Tobasco peppers that my mom gave to me from her garden. I dried a bag full and stored them in the freezer. It only took about a half dozen peppers to make it really spicy. The picture is below, with before (right side) and after (left side) shots. I used all organic ingredients to avoid any possible chemical contaminants that might interfere with the fermentation. The recipe specifies bok choy or napa cabbage. I used bok choy.
I am looking forward to trying more recipes from the book, Wild Fermentation, by Sandor Ellix Katz. It was much easier than I thought it would be and I am very pleased with the results. A link to the book I used is below that. It is an affiliate link, but the pay to me is minimal and I received no compensation for providing this information.