How can I find all Credit Notes?

There is no specific report listing only Credit Notes

Workaround:

1. Select Reports, Receivables (or Payables), Customer Aged (or Vendor).
2. Select Detail.
3. Set "Include invoices paid in the last" to 999 days (or as far back as you need).
4. Select all the customers/vendors.
5. Add a checkmark to "Include inactive customers"
6. Select OK.
7. Go through the list visually and search for the Source that starts with a "CN-" as these are credit notes.