Plenty of online stores would love to know how you find them. If you run your own store, you can add such functionality to your site with the Where Did You Find Us plugin for WooCommerce. It allows you to collect referral info from your customers on the Checkout page.
The plugin has customizable text. You can create options for your customers to choose from. It is also possible to change the position from the back-end. The Where Did You Find Us plug uses graphs to help you make sense of the collected data.
This is a commercial plugin by an Elite Envato Author. You can find out more about it here.