Find work. When you first start out, you may have to accept work writing about a topic you don’t find all that interesting. You must keep an open mind and be willing to accept work that may not be in your desired field. However, as you continue to write, you not only learn about more topics, but you also build your reputation. With time, you can be choosier about assignments you want to accept.
I recognize that the list above is short; however, that’s because Amazon affiliate marketing really doesn’t require a specialized theme in order for it to work well (unless it’s one of the unique cases noted in the themes above). Instead, you’re better off using WooCommerce if you want to build a full affiliate store or simply stick to the WordPress theme you currently have and add affiliate plugins to the mix.