Most extension programs and projects overlook audience mobilization principles to be followed by extension agents to prepare the audience for technological or extension interventions. Such programs also do not consider the knowledge the audience has and its ability to apply or reinvent knowledge, as the social learning theory suggests. Knowledge generation and sharing related to agriculture and its context (i.e., agricultural value chain communities of practices) are handled in the farmers' community of practices, upon which the pluralistic extension concept is based. Such supplies, services, and marketing communities of practices exist, as detected by many studies that explore the sources of farmer information. Most of these studies focused on explaining the results related to "why?" instead of recommending "what is next?" Hence, this study aims to identify the communities of practices related to agriculture through a systematic review of farmers' sources of information that were reported in the published studies. To achieve this, 2852 articles published in many journals during the last 12 years were reviewed, and 60 of the articles qualified for this study were selected, along with 8943 farmers, 704 rural women, 445 livestock keepers, and 1212 extension agents. The keyword "sources of agricultural information" was used to search journals' databases, and then the whole text was reviewed to exclude the studies that did not meet the selection criteria. The geometric mean was used to analyze the percentages of exposure to information sources, and the weighted geometric mean was used to compare the exposure to the different sources.