The dealer can order a code cut key - prices vary from $25 to $50 depending (on dealer, model, etc)
They will want to see your pink slip in most cases, and it could take a few days to be delivered. You get a nickel steel alloy key that fits the factory code, but if your car ever had to have a lock change its possible that you'll have bought a key that won't fit anything!
OTOH, a locksmith that is good and has the blanks can either take the code off the owners ID plate that came with the car (in the owner's manual but unfortunately most have been lost by now) or take apart the drivers door and get the code off the lock assy and cut one to code. This may cost more than the dealer!
good luck!
addendum: If the locksmith isn't experienced or doesn't have the code cutting abilities for Mercedes of your particular era, you may be SOL. Big cities would be where you are most likely to find a good locksmith for this kind of job.
I suggest you ask the local MB dealer, they will usually refer people to a locksmith that has not been blacklisted (they are out there!) although some dealers want you to bring the car to them (tow it?) so they can get a piece of the action! ($$) and they call a locksmith to come do it after they remove the door panel (at $125/hr/)
PS Where on earth did you find a 300TD w/ 4 speed (stick?!)