You won’t get it in Gunns anyway. I asked before myself. They can order, but it’s pricey.
Truth be told they’re fairly pricey anywhere anyway, unless you get a secondhand one.
There was a guy who gave up looking for one and bought a second hand welders mask instead (cheaper too) removed the glass and cut it down to cover his lens. Got some great results. He reckoned it was above 10 stop too!
The “Strongest” Cokin ND filter currently available is the ND8, they briefly produced stronger ones, but they had difficulty with the process and discontinued them. The best option for an extreme ND is to get a Hoya (Screw-in) ND400x which is a 9 f-stop filter.