Holden spare parts will sell them. The part numbers are in that thread I linked you to, but you don't need to buy the bottom of the airbox, you just cut a bigger hole in the original one with a hacksaw or dremel.
As far as cost, I think it comes to about $50-55, and add about $20-25 if you buy the bottom half as well.
Now that I think about it I think I might have bought the bottom half as well on my 1st car, and it came to about $80. But seriously, it is so easy just to cut it open that you would be crazy to pay the extra.
I'm pretty sure Holden could sell you little plastic screws/clips to hold the snorkel on, but I've always just used bolts and washers that are lying around.
There are a million threads about cai's already so it could be worth having a quick squiz at some of the old ones to find heaps of info