I have certainly proven to the world that I can live happily unconcerned about appearances, but that sagging canopy collects rainwater and it all gets rather messy. Say what you wish about the good people who make the Christiania, but they do not really bust their asses off trying to sell their products or accessories. They have not even bothered updating their web page since 2001 and their closest representative can't be bothered replying to emails. The only way I can get new hoops is by travelling to Denmark or sourcing some aluminum retailer around here.
I decided to try improvising by using some plastic Pex tubes instead. This extremely competent bicycling family built some canopies for their kid's seats using Pex.
I straightened some parts of the original hoops in a vice, and filed them on one end (above). I then inserted them into the tubes. This way I can use the existing mounting hardware.
The new hoops (above) look much nicer. Not entirely unexpectedly, they are sort of flimsy. The canopy still sagged!
I managed to fix this somewhat by sewing velcro straps to fit on the hoops. This way the canopy itself functions as a brace, keeping the hoops in place.
All in all, it's ok. But I don't really recommend Pex tubes for projects of this size.