Your best options are probably the specialist mercedes breakers. I've used three pointed parts quite a few times and always had good service.
Carl at w140.co.uk would easily be able to find you the part you need, but does charge a bit more than most parts places. At least you'd know you were getting the right box for the car though.
If it's a 4 speed box you'll need a 722.370
If it's a 5 speed box you'll need a 722.620