You need to view/download your Heroku invoices.
Personal accounts can can access invoices by going to Account Settings > Billing tab. If you don't see the correct invoices, make sure you are logged into the Heroku account you wish to view invoices for.
Click the month to open the invoice, or click See more to see a list of older invoices.
If you do not see the team name on your drop-down, your account is not a member of the Heroku Team; if you see the team name but do not see a "Billing" tab, you are not an Admin of the team. In either case, you will need to speak to a current admin of your Team to make you an admin as well.