Cargo ship travel agencies

Name	Link
Kapitän Zylmann GmbH	https://www.zylmann.de/en/start/welcome.html
Internationale Frachtschiffreisen Pfeiffer GmbH	https://frachtschiffreisen-pfeiffer.de/en/
ATPI Hamburg GmbH (in the past Hamburg Süd Reiseagentur)	https://www.hamburg-frachtschiffreisen.de/
Ship'N'Train Travel (in the past Globoship)	https://www.shipntrain.ch/

It is best to book well in advance as passenger cabins on cargo ships are limited and the most popular routes sell out quickly. Plan six months in advance to ensure the desired trip.
Be aware of the sometimes quite high cancellation fees!