There are several agents who sell freighter trips. Some of them promote one-way streets, but even if they do not, you can always ask. In any case, you need to contact them, explain your needs and see if they can find a ship for you. When you are ready to book, you will need a payment and some paperwork (usually a medical form). It's not like a cruise ship with a fixed schedule or a flight that you can book online in minutes.
Here are some of these agents:
- Maris Freighter Cruise is based in the USA. I have not seen any US-Europe cruise on their website, but I'm sure she could arrange one for you.
- Captain Zylmann is based in Germany and lists a ship in which you can book a cabin from New York to Bremerhaven or Rotterdam.
- Langsamreisen.de is also based in Germany and offers trips from New York to Spain or Italy.
- The Cruise People is based in the UK and offers something that I have not seen in other agents' brochures / websites: tramp travel with a bulk carrier (the other trips are on container ships). "Tramp travel" means knowing that the ship will take you to a location in Europe or North Africa, but you do not know exactly where you are. They book a cabin and know that they are flying from the Great Lakes region to Europe.