The 2001 Buick Lesabre battery is located underneath the rear passenger seats, near the middle of the bench.
To get to the battery, all you need to do is lift up on the back seat. There are no buttons or latches, so it should lift right up when you pull it—you might have to use a little elbow grease to get it loose, though.
Keep in mind that you don’t actually need to reach the battery to jumpstart a 2001 Lesabre. Instead, you’ll just connect the cables to the jump points under the hood—they’re located on the right hand side of the engine bay, next to the fuse box.