Q. Advertising agencies

Sabi (Spanish companies)

• Choose Expert Search.
• Click on Activity in the left navigation menu.
• Click on US SIC.
• Select a US SIC (7311 for advertising agencies).
• Click on Ok.
• Click List at the top menu.

Orbis (Companies worldwide)

• Choose Expert Search.
• Click on Industry in the left navigation menu.
• Click on US SIC in Activity classification.
• Select a US SIC (7311 for advertising agencies).
• Click on Ok.
• Click List at the top menu.

ThomsonOne.com (Note: this resource only works in Internet Explorer)

• Click on Screening & Analysis in the left navigation menu.
• Select a SIC code equal to (7311 for advertising agencies) in Industry Classification.
• Click on Search and execute.

Check out AdBrands.net.

Vault.com - List of Best Advertising Agencies
Ad Age - Agency Report  

Adbrands.net Ranking of agencies by revenues.