Where is the fuel filter located?

It depends on when your Buick was built. Most of the 2005 and later have the fuel filter in the tank as part of the pump. If you have the very early build, they have a fuel filter underneath the car next to the fuel tank between the tank and left frame rail that you change if it's actually showing a restriction which is rare because those also have a filter in the tank.
https://www.2carpros.com/articles/how-to-change-a-fuel-filter