In that case, you can try to initially find at least somewhat related patent document. When you find a patent that is in the area you are focusing on, you may look at its cited and citing prior art references. Each patent cites several other patents in the same field that were considered during the patent examination process. These citations can link you to multiple on-point patents. Many patent search engines include other patents that list that patent as a citation as well (citing references). This can help you collect a large number of relevant patents.