You can get to the Leads By Referral Source Report by going to Reports > Leads > Leads By Referral Source.
This will then bring you to the screen below:
RED - Choose the start and stop date for the Report.
ORANGE - Choose the branches (if your company has multiple branches) here.
YELLOW - Don't forget to click "View" to see your report!
GREEN - These button will allow you to either Export to PDF or CSV file.