It is important to have a mixture of sources. You'll want at least one data source specific to your industry. I'm in healthcare, so at least one data source has to be specific to healthcare. Mercer has different industry specific data sources. It is also important to participate in these surveys. They need data to be applicable. 
You will want another data source that employees access easily. Indeed, or Salary.com. This will provide you the data to support your position in compensation. If employees are seeing one rate and you have it listed differently, you can consider the reasons and adjust if necessary.