Hi,
I also have a CLS55 with a cracked lens on one of the Fog Lights (Left hand side), and the MB EPC only shows them as a complete assembly, no separate lens I am afraid.
I am probably going to replace both of mine with a pair of Osram LED DRL/Fog lights they are not a direct replacement so the car/lights have to be modified to fit.
Peter - Whitenemesis has them fitted to his CLS55 and they do look the part.
For Info MB Pt. No. for the front AMG Fog lights
Fog Light, Left OR Right - A 230 820 05 56
Retail Price from MB Dealer £111.00.
Found a couple on E-Bay, Nebelscheinwerfer original HELLA (1N0 008 582-001) links/rechts 12 V | eBay Mercedes-Benz Right/Left Side Fog Lamp Light HELLA 2308200556
Just had look online ecp price with 30% discount come to 76.29
but i managed to find it a bit cheaper on ebay
new hella light for 58 inc delivery which isnt too bad
let me know if you want link