I purchased mine Aug of 2016 from MirrorGlassPlus.com ; 71 A body Driver side was #2125 (covers multiple years and cars I think). Had both of them installed at local glass shop for about $20/side.
This is from the Burco mirror catalog, 1970 Pontiac Firebird (they don't list Buick) http://burcoinc.com/redi-cuts-catalog/ Measure your mirrors.
Local shop cut mine. If your picky have a decal (sign ) shop. Duplicate the tiny date code then stick on mirror