The GM part no is 385983. That part number was used on 1965-66 Buick Skylark with automatic, non-tilt column. Was not able to find any listings for this part (new, reproduction, or used).
Some similar adapters are being reproduced. A steering column rebuilder dealing in muscle cars might be able to supply this part. Application data shown by some suppliers is suspect. It does not agree with the original GM master parts book references I used.
The 1964 GM Chevelle, Skylark, Cutlass intermediates used a part no 383873 adapter which mounted to the column with a snap ring. Chevelle continued to use it in '65 and '66. This one is being reproduced. My guess is it would be a long shot to adapt it to fit your car.
Steering Column Bearing, lower, new, for non tilt column, This assembly installs into the mast jacket like the original and attaches to the mast jacket with wire clip. No modification to the steering column is needed to install this assembly. This assembly replaces GM #383873 adapter and 7802845...
www.chicagomusclecarparts.com
The Cutlass for '65-66 used a part no 385984 which is also being reproduced. This one bolts to the column. You might be able to tell if it could be modified for the Buick.
Steering Column lower bearing, new, for non tilt columns. This assembly replaces GM adapter # 385984 & bearing # 7802845 . This assembly installs into the mast jacket with 2 bolts, like the original and attaches to the mast jacket with 2 small stainless steel screws that are included rather than...
www.chicagomusclecarparts.com
Here is a reference for a column rebuilder who indicates they have adapters available. The 2040-parts.com site appears to be defunct as the listing is very old. However the rebuilder CJ's Rods and Machines Pfafftown, NC (336) 922-1873 appears to still be in business.
Buy 64-66 Chevelle & Impala New Lower Steering Column Bearing WO/Tilt, 100% USA L@@K in Pfafftown, North Carolina, US, for US $65.25
www.2040-parts.com