There's Ram Fabrication in Bibra, They do alot of civil and minning stuff, but at the end of the day, they all spend their dough on race cars. All custom.
Also a bit further is Cannington Performance near Carousel, my brother in law got an exhaust there, mainly stocked and ordered items.
if you can be bothered, pete at exhaust world on newcastle street. really good prices, really close to the newcastle/charles st freeway exit so you dont have to worry about actually driving in northbridge. has experience with modified cars and will price better than most shops around.