Preqin

• Click the "Search for" tab at the top of the page
• Hover over "Investors" 
• Click "Advanced Search" 
• Click on "Investor Type" dropdown, and choose "Family Office - Multi" or "Family Office - Single" under Institutional Private Wealth. 

Capital IQ 

• Mouse over the "Screening" tab
• Select "Companies" from the drop-down list
• Within the "Investment / Advisory Firms" section, select "Investment Firm Type"
• Then select "Family Office/Family Trust", and click "add criteria".
• Add other screening criteria as desired (geographic locations, etc). 

Pitchbook

• Hover over ​"Investors & Funds" from the left hand side navigation.
• Click "Build a custom Investor & Fund screen"
• Click "Investor Type"
• Select "Family Office" under "Other Investor Type"

CB Insights

• Click on "Search" and then "Investors" in the left side navigation menu.
• Click on the "Investor attributes" menu, then select Family Office under "Investor Types". 

MassInvestor (US focus)

• Go to "Select a firm type"
• Click "Family Investment Office"

Crunchbase (In-library access only)

• Click "Advanced search"
• Select "Investors"
• Expand investor options and select "Family Investment Office"

Crunchbase also offers a "Diversity Spotlight" filter for US headquartered companies. Includes filters for owner/founder by certain affinity groups.