code: def find(self, source, issuer):
          
          try:
              for kb in self.issuer_keys[issuer]:
                  if kb.source == source:
                      return kb
          except KeyError:
              return None
  
          return None
docs: Find a key bundle based on the source of the keys
  
          :param source: A source url
          :param issuer: The issuer of keys
          :return: A :py:class:`oidcmsg.key_bundle.KeyBundle` instance or None