Call Tom Hanson at Caliber Motors in CA, an MB Dealer (I don't have the number, try 800 directory assistance). You may need to fax him proof of ownership, etc. I got a W124 ingnition cylinder and key ('86 300E) from him for about $70 + $8 FedEx shipping. The package comes with a spare key because they set the lock to the key. Installation is very simple, if your key still moves. I posted the entire service manaul description a few posts back -- search for it. It involves making a very simple wire tool to pop the tumbler out. The whole thing takes about 10 minutes once you've done it. This should be cheaper than rekeying the other locks on the car!
PS You can then sell your used, non matching tumbler and key on eBay.
1986 300E Anthracite + ECodes + MB Mileage Award