Query To Get Payment Method for Supplier and Supplier Sites

This information is stored in IBY_EXT_PARTY_PMT_MTHDS which joins to IBY_EXTERNAL_PAYEES_ALL by ...
IBY_EXT_PARTY_PMT_MTHDS.ext_pmt_party_id = IBY_EXTERNAL_PAYEES_ALL.ext_payee_id.

This table holds any payment method that has been the default for the site. The current default
will have IBY_EXT_PARTY_PMT_MTHDS.Primary_Flag = 'Y'.

You could use these queries to see the rows for a given supplier site:

This one will show the default payment method (if any) at the Site level:
  SELECT SUP.vendor_name SUPPLIER,

         SITE.vendor_site_code SITE,

         SITE.vendor_site_id,

         SITE.org_id,

         IBEPPM.payment_method_code

    FROM ap_suppliers SUP,

         iby_external_payees_all IBEP,

         iby_ext_party_pmt_mthds IBEPPM,

         ap_supplier_sites_all SITE

   WHERE     SUP.party_id = IBEP.payee_party_id

         AND IBEPPM.ext_pmt_party_id = IBEP.ext_payee_id

         AND SITE.vendor_id = SUP.vendor_id

         AND IBEP.supplier_site_id = SITE.vendor_site_id

         AND IBEPPM.primary_flag = 'Y'

         AND SUP.vendor_name = '<Supplier Name>'

ORDER BY SUPPLIER, SITE;

This one will show the default payment method (if any) at the Supplier level:
SELECT SUP.vendor_name SUPPLIER, IBEPPM.payment_method_code

  FROM ap_suppliers SUP,

       iby_external_payees_all IBEP,

       iby_ext_party_pmt_mthds IBEPPM

 WHERE     SUP.party_id = IBEP.payee_party_id

       AND IBEPPM.ext_pmt_party_id = IBEP.ext_payee_id

       AND IBEP.supplier_site_id IS NULL

       AND SUP.vendor_name = '<Supplier Name>';

This will show all Payees associated with a given supplier. The payee associated with a Supplier site will have a value for Supplier_Site_ID

SELECT SUP.vendor_name, PAYEE.*

  FROM iby_external_payees_all PAYEE, ap_suppliers SUP

 WHERE PAYEE.payee_party_id = SUP.party_id

       AND SUP.vendor_name = '<Supplier Name>'