Where can I get a spare tire for my 09 GT

You can order one for a G8 V6 direct from your dealer. It has a part number so must be available. Or you can buy one imported by a reseller along with jack imported directly from Oz. There is more than one outfit importing Holden parts but I won't mention their names in case it isn't kosher with advertising rules.

help how do i find out which # g8 i have purchasedit may 2008

I've been told by the dealer the spare for the V6 will not fit on the GT because the rotors are bigger. So I need one to fit a GT.

I have seen in various forums that a normal 5th tire like the other 4 will fit in the space with a basic steel wheel. I plan to use one of the original Bridgestones as my spare when I put all seasons on next year. The spare tire jack kit can be had at Pace Performance products in Niles, IL.