Dog box gear kit, shift forks etc are purchased through Albins gears, Pfitzner gears, Modena Engineering among others. Then they are installed in your existing case with your existing transfer gears, Final drive gears and diffs. 5 speeds run to about $8000 by the time you set it all up. 6 speed dogbox is approx double. Essential for racing but not ideal for street use and unable to be engineered for street use (in W.A. at least)

6 Speed synchro would be a better option for street use. There's a number of wreckers and importers that sell them, or scour the forums. I no longer sell them unless I do the whole job of installing it as well.