HeatherQ- If you edit the Customer List screen you can add the LeadSource drop down to the screen. By default Method doesn't have any reports on lead source. Based on the kind of report you are loooking for you would need to link the LeadSource to the Invoice table based on Customer and then you can create a reports on the invoice table and have access to the lead source.
I am going to suggest watching our webinars HERE, for information on linked fields and creating reports.
-Michael