Any fire or saftey store, usually medical or safety supply stores, I have never seen it in the hump but I have seen them on roll cages mounted, I have seen on mounted on the roof line near the driver also
Do not use a dry chemical one. If you ever have an electrical fire and use a dry chemical one, it may put out the fire but it will corrode everything it touches. And it is really hard to clean up.
Try one of the halon replacement ones.
Halon or an equivalent is the best. Only problem is you must get close to the base of the fire to use them. They are much more effective and of course expensive too.
I have a 10 pound Halon rolling around behind my driver seat (Rarely does anyone sit in the back).
Halon and the equivalents will not corrode anything...other than some GREEN WACKO worried about the ozone....but I think some of our cars burn more and harm... then than occasional Halon discharge.... EPA be damned!
Home Depot and Lowes have a line of extinguishers in all classifications, usually an ABC is best for home use as it covers wood,trash,combustible liquids and Halon for autos as they snuff out the oxygen so the fire cant burn.