Most prior literature uses beneficial ownership by officers and directors, which is reported in the proxy filings. This measure can be constructed by merging director ownership data from RiskMetrics Directors data, with non-director executive (officer) ownership data from Execucomp (Compustat Executive Compensation). Execucomp has a field - EXEC_DIR - to identify whether the executive is a director or not.
Note that, one might be as well interested in blockholder ownership (5% or more) as it is sometimes considered as insider ownership (especially if ownership exceeds 10% - refer to SEC Form 3 info: https://www.sec.gov/answers/form345.htm). Blockholder data can be found on the following page, Blockholders and is available free of charge for all WRDS users.
Thomson Reuters insider data is not appropriate to construct a measure of insider ownership. This is in part due to the nature of insider filings (SEC Forms 3, 4, 5 and 144). For a nice comparison of various ownership data sources, refer to Anderson and Lee (1997) ("Ownership Studies: the Data Source Does Matter", JFQA, 32, 3, Sept 1997).