Have you checked the manufacturer's website to see if replacement parts are available? Should be easy enough if you have the lamp number. If the lamp belongs to you, then you would be responsible for getting the parts. However, if it is a permanent fixture of the flat, then it is on your landlord. Good luck.

An IKEA lamp probably has the same size shade retaining ring in it.

This is the part we need (we have two lights in our flat and only the one has this part)