where is the battery

Hello,
I have not been able to find a picture of one online.  However, there should be one in your owner's manual.  If you still cannot find it, look for a rectangular object with a red wire and a black wire coming off of it.
Best of luck!
CarGrrl

under the seat

could it be under the backseat ??? That is where mine is in my 1998 Riviera

The battery to you Buick is under the rear passenger seat. If you put your fingers between the seat itself and the back rest and pull outwards it will "pop" out. There will be the battery under the seat. If you replace it make sure not to discard the rubber tube that connects to the battery. This vents to gases produced the battery to the atmosphere. I would jump the vehicle from under the hood and drive it around to fully charge the battery before replacement. The positive jump terminal is on the passenger side under the hood. It is connected to the fuse box and has a red plastic cover over it. A good ground would be anything without and paint on it. Hope this helps.