You can view and edit the sources that appear for your clients by logging into Partner Center and selecting Administration > Customize > Listing Sources.
The selected sources will be available for all accounts created on your platform. You can also remove sources on a per account basis by logging into/impersonating an account and editing the selected Listings in the settings tab.
View Vendasta's Listing Sources by downloading the attachment below. Please be advised that these sources are subject to change at any time.