Below are some useful links (or rather, ... links that I found useful). 

Firm Level Data

• Anual Survey of Manufactures (Census Bureau)
  

• Business Dynamics Statistics (Census Bureau)
  

• Doing Business (World Bank)
  

• Enterprise Survey (World Bank)
  

• DANE (Colombia)

• Encuesta sobre estrategias Empresariales (ESEE,Spain)
  

• Confidential Census Data  (Guide to apply by Mathias Kehrig)
  

Sectoral Level Data

• EU-KLEMS 
  

• The Argentinian Version, ARKLEMS 

• Input Output Matrices in the US (BEA)

• Annual IO Matrices for the US (BLS)