Just replaced my fog light on c class w204 amg bumper. Could not get the glass on its own, but found a used genuine one on ebay £20.
Be wary of after market, I tried 1 from Euro parts, 1 from Ace parts ebay and one from Gumtree. The non genuine aren't quite right and do not line up correctly.
Try Dronfields, but I found a lot of places wanted to sell the fogs and bumpers together. Mercedes Leicester quoted £97 for one fog light.
Mines had a crack on the glass and a pin hole sized hole, manageed to sell it on ebay for £10 to someone with a totally smashed glass on fog light.