Important Note: The location might be slightly different depending on whether it has a 3.8L or a different engine, and whether it's a 4-speed automatic or a different transmission. A picture or a more detailed description of your LeSabre's engine would help narrow it down. However, it is almost certainly on the transmission case itself.
If you can't locate it, consulting your owner's manual is the best course of action. Failing that, a picture of your engine bay might help someone more familiar with that car model help you pinpoint it.