You have a couple of options if you want to buy iTunes gift cards online.
Most people eventually make their way to the actual iTunes website, scroll down to the bottom, look to the right, and under the iTunes Store listing there is a link that you can click that says Buy iTunes Gift Cards.
You won't find any discounts most of the time though. Sometimes they do make special offers, but the popularity of these cards is such that they don't have a need to do that.
For those who like to find the best deals, save some money, and get their gift card purchases at a discount there are a couple of other sites that deal in iTunes gift cards.
Plastic Jungle sometimes has these cards discounted, but they sell very quickly when they do become available. Most of the time you will find that you have to put your name on a waiting list to be notified when they get some in.
Gift card discounters have a pretty neat business model because lots of people are looking to trade in gift cards for cash, or are looking to swap them for another kind of card. It is because of this that you can find some pretty great deals on a variety of different cards.