Many companies pay to access large sets of data called compensation surveys. These surveys are organized by companies that collect data from many organizations and compile it into searchable datasets. This data is usually reliable, as it is collected from the companies directly.
You may also choose to reference self-reported and/or crowd-sourced data on websites where many individuals submit their compensation information (such as salary.com, indeed.com, glassdoor.com, LinkedIn.com, etc.). While this data is potentially helpful, it may be less accurate depending on how many data points are gathered and the accuracy of the data submitted.