Tertiary sources include dictionaries, encyclopedias, almanacs, fact books, bibliographies, directories, manuals, handbooks, and indexing and abstracting sources. Try searching in OneSearch. For example, encylopedias AND [your topic]
Many of your information communities will require some creative thinking when it comes to finding scholarly sources for your paper. For example, it is unlikely that there will be many peer-reviewed articles on Persian cat fanciers specifically so think of this group in a broader context (hobbyists, pet owners, pet ownership). If it is a group that socializes only online, "virtual communities" is a possible search term. As you can see, you will need to draw parallels and extrapolate in order to find the peer-reviewed articles you'll need. Below are the best resources to start your research.