The seal shows up in the Buick Master Parts book as no 1234498. A GM supercedure caralog shows it changed to 1255938 about 1/82.
Www.rockauto.com shows they carry a Timken seal no 712146 that is the same as the Buick nos. It doesn't turn up by searching Buick applications. Their site seems to revert to the 12-bolt Chevy type "C" clip-in axle types or a Pontiac type.
Www.buickfarm.com shows to have the seals under the GM nos.
Regular bearing supply houses like Motion Industries should carry a fresh stock of the Timken seal.
There are plenty of examples on line by searching the web using the GM nos. It might be good to check the age of these if possible. Some items might tend to leak prematurely from old age.