What to do? After looking online for hours, we honestly could not find anything available that was a proper solution (in the area), one advice was even to buy a 28mm sleeve, which obviously wouldn't fit, duh.
So, it's DIY time. After looking around in the hardwarestore, we came up with this:
duct tape (of course!)
2 round pvc pipes, 40 mm
2 pieces of pipe insulation, 28 mm
First, saw the pvc pipe through lengthwise, and slide in the piece of pipe insulation. Then pop it on the barbell to make sure it fits properly.
Next, wrap it with duct tape (while still on the barbell) until you reach the desired thickness. Wrap it with a few pieces at a time, and test if the plate fits on. The result:
Make sure you wrap enough duct tape on so that the plate fits a bit tight. The insulation material will lose it's size a bit over time. To be safe, I put a collar at the end of the barbell to keep both the sleeve and plate in their place.
Too lazy? Get this: