Here's a place that lists overdrive relays. Maybe they have more.
http://www.car-nection.com/yesteryear
This guy has overdrive in his ad too:
SPIRAL shocks, $100; batwing air cleaner, $1,400; 57 continental kit, 55 hdtp doors; 57 positrac, $1,350; NOS 57 power antenna, $900; power steering, $350; HydroVac power pac heads, $250 pr; motors, transmissions, 2-bbl & 4-bbl air cleaners, $75-up; intakes, single & 2x4 overdrive, relays, wheels; 55 accy bumper & grille guards; shavers, $300; 55/56 jacks, $150; many other parts. 913-268-5314, KS
And this one:
NOS 55-56 OVERDRIVE relay, $40; remote control mirror, $300; 57 Bel Air grille, $250; 56-57 ring and pinion, $300; 56 accumulator/dryer, $100; used: 56 padded dash center trim, $75; 57 Electrovac (top half), $200; 57 1271 carburetor, $125; 55-57 convertible rear regulators, $200. Alvin Shepheard, 817-498-7877, TX.
Another one:
CHEVROLET 3-spd OVERDRIVE transmissions, 1965-71 cars & trucks, rebuilt, $1,500 fob. 707-939-1993, CA.
Might be a lead.
Tom
P,S, Ads from
www.hemmings.com