Low fares to Dusseldorf from several airports are available through SkyTeam airlines’ own websites with Delta at $397 for Boston to Dusseldorf and KLM $449 for LAX-Dusseldorf in September. This SkyTeam deal is available from Los Angeles, Boston, New York, Philadelphia and Miami for departures from September to December. These fares can be searched through Google Flights. You can try Momondo.com for potentially lower fares once you have found suitable dates, but the fare differences I found were negligible.
KLM Los Angeles – Dusseldorf DUS
Delta Airlines Los Angeles – Dusseldorf, Germany DUS
Delta Airlines Boston – Dusseldorf, Germany DUS