The part number is 25556970 for the side emblems. Here's one on E Bay,
http://www.ebay.com/itm/1995-1999-Buick-Riviera-Quarter-Panel-Riviera-Nameplate-Emblem-new-OEM-25556970-/271739212331?fits=Model%3ARiviera&hash=item3f44eb522b&vxp=mtr
Here's one that's cheaper
http://www.ebay.com/itm/OEM-1995-1996-1997-1998-1999-BUICK-RIVIERA-FENDER-EMBLEM-CHROME-/161825174654?hash=item25ad88347e&vxp=mtr
GMpartsdirect,
http://www.gmpartsdirect.com/oe-gm/25556970
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98 Riviera SC3800 All stock except gutted air box.
1970 Buick GS455 Stage1, TSP built 470BBB, 602HP/589TQ
Best MPH, 116.06 MPH, Best ET, 11.54
https://www.youtube.com/watch?v=UHCda-t_Jls
https://www.youtube.com/watch?v=sfT2tEO4XcU