Looks to me like you have three options --
first one are these guys, whose canoes I see whenever I'm paddling on the Ruhr (and that is their river, nothing fancy, but nice enough)
Startseite - Lenne-Ruhr-Kanu-Tour
(do you actually speak German?)
Second, there's a school that might not rent (I'm not entirely sure, it doesn't say so on their website) but they're certainly connected and they have access to some amazing material (the boss also works for Lettmann, a German kayak producer). Also, they're closer to Düsseldorf than the Ruhr River is, and might well know some other options
If I'm around, and have time, we could get boats from my club. Most of our kayaks are rather ancient and not exactly the pinnacle of comfortable outfitting, but they swim. We're located in Essen, so it's a 40 minute drive from Düsseldorf, or an hour by train, tram, and then foot from Düsseldorf main station.
Hope this helps a little