What you are looking for is your marketing avatar. There is a process to develop your avatar that I go through with my clients. Before you can figure out how and where to find those people online, you must know who they are. The answer to the second part of your question can be answered after the first part is determined. Building the persona comes from someone skilled in eliciting that information from you and if available, from your marketing analytics. Once the avatar is created, then the fun begins!