Oops - posted on a specifically 61-63 Special/Skylark forum with not much traffic. Forgot to add additional info on here. Per my avatar, 62 Skylark - Dual Path Trans.
Presumably this is the piece which bolts on to the mainshaft at the rear of the transmission. The Buick parts books call it the front companion flange, GM part no 5676065. It was used on late '61 and '62 compact Buicks. Later the '63 part no 5677182 superceded the late '61-62 part. Searching by these numbers and description, nothing came up.
The only places I can think of to try are transmission parts suppliers such as:
You could also try an antique trans. supplier called FATSCO in N.Y. as another possibility. I'll tell you upfront they are not cheap, BUT do carry MANY obsolete parts.