Washer is GM 25519277 and the nut is GM 25519219 if you need the grommets they are GM 25518660 or Felpro ES72134
This is what to look for, where the nuts and washers were used besides our TTA's, either online or if you have a junkyard close by:
Buick Century 1986, 1987, 1988
Buick Electra 1986, 1987, 1988
Buick Lesabre 1986, 1987, 1988
Buick Reatta 1986, 1987
Buick Riviera 1986, 1987
Buick Skylark 1985, 1986, 1987, 1988
Buick Somerset 1985, 1986, 1987, 1988
Oldsmobile 98 Regency 1986, 1987, 1988
Oldsmobile Calais 1985, 1986, 1987, 1988
Oldsmobile Cutlass Ciera 1986, 1987, 1988
Oldsmobile Cutlass Cruiser 1986, 1987, 1988
Oldsmobile Delta 88 Royale 1986, 1987, 1988
Oldsmobile Toronado 1986, 1987
Pontiac Bonneville 1987, 1988
Pontiac Firebird 1989
Pontiac Firebird Trans Am 1989
Pontiac Grand Am 1985, 1986, 1987