Have a dealer do a search. Believe it or not, they are out there. They lasted so long that some dealers still have one.
master parts book says... grp. 2.895...64-66 lesabre-wildcat-electra-riviera tilt whl...1993621 i dont see a non-tilt for a riv. .. or anthing about cornering lights. what all does your car have? i dont see that part number in the olds master parts catalog. both books cover up to 1975.
after some searching, it looks like some caddys use it. 2.895 1993621 SWITCH ASSEMBLY, DIRECTIONAL 1963-1964 (TILT WHEEL); 1965-1965 EXC 75 (T&T); 1966-1966 (T&T); T&T i think is tele tilt.
Tilt wheel was standard on 66 Riviera, and cornering lights were available, which requires a different turn signal switch.