Some major components can be horrendously expensive when new and a good used part at a fraction of the cost often makes economic sense. Here's a listing of places that sell used parts.

These vendors will sell you the same OEM parts that Mercedes uses - they just don't come in a blue box with a star.

Somewhere between new and used are the rebuilt parts. Here is a small listing of vendors

Some things for older cars are no longer available and after-market suppliers exist for some of these.