Beige is a tougher color to match. . . I found a passenger seat back on eBay, likely MbTex.
If you removed your seat you may be able to swap out the seat back for about $300. Not sure how much down time would be involved.
What I liked about my seat swap project was that I still had the original seat as a back up. I have not seen very many complete W204 driver seats on eBay. The ones I have seen are usually sold as a complete interior.
I absolutely love my 2012 C350. Reliable with minimal maintenance. I can get 36 mpg on the freeway. I just turned 100,000 miles, if I could get another 100,000 I would be happy. I plan on keeping this car as long as I can. If my back bolster was going bad I would either
1) Keep searching eBay for a complete seat to swap (real leather seat preferable)
2) Research what is involved to swap out the seat back (You Tube has everything)
3) Get a quote from the upholstery shop.
