There are usually a couple on ebay. I paid £45 for one for my SL . The glass unclips, the electrics unplugs, in with the new, job done.
It really is easy. Try this one, but check part numbers first!
Take a look at the link below and see if yours looks the same as the one I recently replaced on my C Class.
Must warn you that new replacements are £300+ from the dealers, but I picked up a used one in 'as new' condition off ebay.
The alternative is to replace it with a standard heated glass. Non dimming.
Mine went brown.
I split the two glasses and cleaned the oily stuff out, then bonded back together around the edge.
I can live without auto dimming until a cheap one comes my way.