I forget what it cost when I got a key for my 91 from the local dealer. I want to say it was $40 or $45.

Probably your best bet. It’s a steel key, I’ve seen locksmiths machine laser cut key equivalents. But from brass.

I’ve also seen aftermarket ignition tumblers and keys but then you have two keys, not one. Not sure if/how the ignition tumblers get keyed to the laser cut keys.

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Current Diesels:
1981 240D (73K)
1982 300CD (169k)
1985 190D (169k)
1991 350SD (113k)
1991 350SD (206k)
1991 300D (228k)
1993 300SD (291k)
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1996 Dodge Ram CTD (265k)

Past Diesels:
1983 300D (228K)
1985 300D (233K)