The gift cards don't need any address to use, so it doesn't really matter where you are.
As for distinguishing them: I've not actually seen one, directly (so take this well salted), but I understand they say something like "Sony eBook Store Gift Card" or some such. If you can get into a Borders and ask for gift cards for the Sony eBook Store, they folks there ought to be able to point you straight.
As an aside, I just want to make sure that you realize that you can buy books from a few other places (like Baen's Webscriptions.net and Fictionwise.com), and that you can convert from several other formats with just a bit of work. That opens things up quite a bit beyond the Sony store.
