You can access all your billing and invoices for the duration of your program going to the Settings page, then selecting "Billing" from the dropdown.
On the billing page, you will be able to view all the leads that you have received broken down by the ID, date delivered, price, lead type and the name of the lead.
You will also be able to use the filtering at the top of the page to view historic leads for the duration of your program and download them in either PDF or CSV format.