Take the title to any MB dealership and they'll order you a key by VIN. It was $37 including tax when I did it last year for my SDL. The ignition locks are a common thing to fail and get replaced with generic parts.

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]