You have to insert this cheat code in your in-game phone by pressing UP again on your keyboard or controller:
227-555-0100
(CAR-555-0100)
Here is a video on how to use cheat codes in GTA 4 as I never used them myself.
PS: All cheats for GTA 4 work in the Episodes too.

The FIB Buffalo has a chance to spawn when you have a 4+ star wanted level. I say chance because most of the time you will see NOOSE chasing after you instead of the FIB agents.

If you do not want to build up a wanted level,you can easily get a 5 star wanted level (4 if you didn't unlocked the islands) if you visit the forbidden website on the in-game internet:


Or if you are really good at evading the cops and know the map well,you can try it at the very start of GTA 4 when none of the island are unlocked,as you will get a maximum of 6 star wanted level or if you shoot a NOOSE officer guarding the bridge.

The following missions in GTA 4 and Episodes will always have a FIB Buffalo that you can grab for yourself:
GTA 4-To Live and Die in Alderney
When you get to the abandoned house,instead of following Phil,immediately get out of the car and steal one of the parked FIB Buffalo,you have to do it quickly as soon you will fail the mission and it will trigger a cutscene,if you were in the Buffalo while the scene triggers,congratulations,now you can keep it for yourself!

GTA The Lost and Damned-Bad Cop Drop
You can get one when the agents get out of their cars to shoot you,finish the shooting,get out using an FIB Buffalo and gg. Unfortunately you cannot save it,because you are forced to ride bikes pretty much this whole DLC.

GTA The Ballad of Gay Tony
After the shootout in the underground parking lot,when you are told to leave,after the NOOSE truck,2 FIB Buffalos will spawn and steal either one of them,complete the mission and done.
Note that I don't know if there are any side effects when you replay a mission in this DLC so if anyone knows,leave a comment below.

If you use a trainer or a savegame editor,you can spawn it directly in your face but unlike the cheat code,it won't disable your 100% progression or achievements if you care about those,but be aware that your save file might get corrupted,so use any of those methods at your own risk.

I hope this guide helped you in some way,and enjoy this rare vehicle.
Constructive feedback is welcome in the comment section.
Credit goes to Badinfos for his well done videos showcasing and explaining each part about these missions and cheats.
(If by any chance Badinfos you stumble upon this guide and wish these videos removed,just DM me.)