Where does the in take vacuum line above brake booster on a 2002 Buick lesabre ho

open hood and look up, see EPA map,? vacuum map, sticker.

or get the service manual
add brake booster to list , booster is not smog related, so not there.
all else is, EGR,MAP,IAC,,and more.
or READ the SM service manual
here
https://eautorepair.net/Marketing/Default.asp

Vacuum leak from hoses or the vacuum canister leaking.

A leaking Power brake booster can cause you to loose enough vacuum to cause a problem also.

disconnect the vacuum hose to the booster, then disconnect unbolt the master cylinder from the power booster(2 nuts) remove the cylinder from the booster by sliding it off the studs. inside the car under the dash disconnect the brake pedal from the booster rod and undo the booster mounting nuts/bolts there is 4 or them.

May not be the booster as some do make a noise when the brakes are applied. To test press the brakes several times with engine off, Press the pedal down hard and start the engine , the pedal should move down slightly and there should be no whooshing sound . if the above is ok so is the booster.

Has to go from the booster to a vacuum source
on the intake--- somewhere

Google--a picture

Have you not followed the hose yourself
to answer that???

These are not rebuild-able, they must be replaced if they leak.
One that leaks will hiss while the engine is running. The hiss is vacuum leaking, not pressure leaking out.
To possibly check it under the hood, be very suspicious of a leaking vacuum line at or near the booster, not the booster itself. It might be bad, but that is pretty rare.
The last one I changed I thought was defective because it hissed when I applied the brakes, but a new one did the same thing! :-(
If you decide to replace it, you will be unbolting the master cylinder from the unit under the hood, and also unbolting the unit under the dashboard.
Of you are really careful, you might not need to disconnect the master cylinder 's brake lines at all, just gently pull/push it out of the way to change the booster.
Be really careful not to kink the brake lines.
The unit is bolted to the car with usually 2 bolts through the firewall behind the brake pedal.

Heres a way to save you a lot of time. remove the two bolts that hold the master cylinder to booster, leave all brake lines connected, pull master away as far as it will go, you may have to pull on the line some, next go inside and remove the pin to remove the rod from brake pedal, you can let brake switch stay, back to booster, remove vacuum line from booster, back inside the car remove the 4 nuts that hold the booster to firewall, make sure rood is clear, go out and pull booster out and remove, install new booster make sure rod hooks to pedal before bolting it up, because sometimes the rod may go in the wrong direction, reverse directions to install, when your done theres no need to bleed brake system. hope this helps.

booster rarely go bad and when they do they make a vacuum sound in the car see if vac hose is going to booster from top back of engine 2 booster if that is got vac . but the leak in the brake line is a bigger danger and you cannot diagnose the master or booster with out fixing that leak first &bleading the brakes

If you look down under the radiator in the area of the power steering pump, nestled in the crossmember you should see some brake lines going in and out of a rectangular steel block. It should also have a single wire going in the top. That is the proportioning valve which adjusts brake pressure to the wheels. One of these lines is likely the source of your leak. Hope this helps.

if you can see a good amount of wet brake fluid between the brake master cylinder and the brake booster. and you have been adding fliud often,you could have a bad master cylinder. it has to be replaced not rebuilt. you however may have a far more common problem, a rusted through brake line.have someone pump the brake pedal, than look under the car for a brake fluid leak ,repair as needed. than bleed the brakes for a good hard brake pedal
good luck chris