I bought my superb with fogged headlights, I took them to get polished but they're still not as clear as I want them. A quick fix would be changing the lenses but I haven't found any site that sells them. Did anyone here change theirs and where did you get them from?

I've never seen the lenses available on their own, only whole headlight units.  New will cost a lot, and second hand ones will likely be no better than your current ones.

 

To properly refurb the lens they need more than just polishing.  A fair depth of the surface needs to be removed to get rid of the cloudiness and down to clear plastic. This involves lots of sanding followed by much polishing.  A small rotary sanding pad on a drill will be easiest.  Search for a 3M headlight restoration kit for about £20. Ebay will find you the kit and youtube will give many videos of the job.