I bought a full set of rubber mats from NP Autoparts on line.
The mats are thick and heavy duty, fit perfectly including the locking clips on the fronts and are around half the price of the MB mats. I've used this company several times for my other cars and have always been very happy
It would help if we knew where you are from. Are you in Canada or USA? You might want to update your profile so people know.
If you are in Canada or USA you should check out the Maxpider 3D floor liners. They are much nicer and less utilitarian than some other brands. They are available thru Costco. Costco item #393507. Maxpider #PB-FL-9012BK. Cost for the set (front & rear) is $190 CAN$.
I purchased them for my new GLC AMG and they fit perfectly and look very nice.
I got some all weather foot well inserts for fronts and back from weathertec. They are custom made for the glc and fit perfectly. If you search for weathertec on the web their website should come up. I put my carpet mats I top of these for normal days. When it rains or the kids are particularly muddy, I take the carpets out and put them in the boot. The mats just hose down when dirty.