The following is a list of locations where the fire extinguisher spawns in San Andreas.
Inside the Vagos gang house in East Los Santos (only during the mission Burning Desire)
Inside Big Smoke's crack den in East Los Santos (only during the mission End of the Line)
Over the counter at the Well Stacked Pizza Co. in Idlewood (found during the mission Ryder)
The wiki is inaccurate or incomplete (as usual). I've done the mission "Nines and AK's" (the 6th mission I believe), the Ryder mission was about four missions earlier.
A fire extinguisher is at Well Stacked and also at both Burger Shot too.
It is not at any Cluckin' Bell restaurant.