Believe it or not, small parts like knobs and switches are available from MB. I got some from calibermotors.com/Tom Hanson for my W111. I got a new Hazard knob for under $4 and a vent knob cover for $13. Kinda pricey but available and in perfect condition with warranty, I don't know what kind of warranty you'll need for knobs though.

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1970 280SE Coupe W111
Chassis Number: 111 024 12
Engine Number: 130 980 12
Exterior Color: 180 - Silver Grey Metallic
Interior Color: 243 - Leather Light Red
Transmission # 004470