We manually analyze our domains' history and backlinks and determine up to 3 categories and sub-categories that best describe the domain. This is, by far, the easiest and most accurate way to find niche related domains.
Please explore all sub-categories and categories. Sometimes the sub-category is not intuitively related to its category. It's also important to read this myth regarding niche SEO-domains.
Note that a small fraction of the domains don't have manual categories.