You can. In fact, you can buy up to 2 pieces of SS Goku. Just preorder him from Sideshow now, wait and hold on to your preorder until it comes out
http://www.sideshowtoy.com/?page_id=4489&sku=900010
I did the same thing with the Comic Symbiote Spidey, without the need to send in my regular comic Spidey's proof of purchase voucher (code).
This has got to do with Sideshow and some other online retailers bringing Medicom stuff and their "so-called" exclusive to the US. It's the only way to get them close to retail price, because after they sold out the ebay prices will be very expensive.