You can get a "data card" from the Classic Center, which includes the codes for all three keys to your SL ... assuming it still has the origina locks. I just got a copy of my data card last week, so they are still issuing them. All you need to do is scan your title or registration to prove ownership and send them the 14-digit vehicle number from the factory plate on your car (NOT the VIN), and they will email you a scan of the data card as well as a key to the options list, paint codes, etc.
The 230SLs had a large metal key (no plastic) for the ignition ... plastic was added later for the 280s (not sure about 250s). Door key is smaller and squarish, while the trunk key is same size but more rounded.
Good advice, Kayvan. I seen OEM key blanks for the ignition keys on eBay, but they are a bit expensive. Ordering from the Classic Center sounds like a better solution.
Brian