It's kind of weird the way they have it set up. There's no specific section where you can go to buy a card; it is just an option you will see when you click the link for a specific book.
Example here:
http://www.kobobooks.com/ebook/Port-...yNQ/page1.html
When you click the link "Send an EGift card" it takes you to a form to fill out with the amount from $10.00 to $100.00.