For the 123, a GOOD local locksmith will have the blanks, and can make a brass copy that will work fine. I do NOT recommend using a brass key as your daily use key, only as backups & spares. If you don't have an original steel VIN-coded key, order one from the dealer (about $25 last time I got one). The brass ones wear and leave a little pile of shavings under the ignition key slot - not good.
For the 124, 201, and most newer MB's that have the "high security" keys, don't even waste time calling a locksmith. They can only make brass copies, and will charge you MORE than it costs to buy a new steel VIN-coded key from the dealer! I learned that the hard way... brass copy was $25-$35, and a new steel VIN-cut key was only $17 at the dealer! Go figure.
BTW, there should be no need to order blanks from the dealer, give them your VIN and you'll get a nice, fresh original key. And keep the brass copies in your desk drawer at home for when you lose your main key!
HTH,