The stores in the actual WWOHP island are all very small compared to what you might be used to in WDW or even the other Universal stores. Because they are so small, they can feel very crowded and claustrophobic even on a low crowd day. We visited this past November 2010, and it was a low crowd time for WDW and Universal - EXCEPT for the WWOHP. We stayed in a Universal hotel for a few days just to get the hour early admission because this was a special trip to see the WWOHP. We used the early admission available to on-site guests every morning we were. (We would even get to the park an extra 30 minutes earlier than the "early-entry" time - you can say we are a little obssessive.) Each time, we went straight to the WWOHP island. Everything was nice and quiet for about 1.5 hours, then the off-site guests arrived and it was packed. In early November! It seemed to be mostly people visiting from other countries, which was super cool to hear all the different languages, just as if we were at the Quidditch World Cup!
Sorry, off-topic a bit, but here's the answer to your original question. You can buy your WWOHP souvenirs in the store at the very front of IOA (it's called Islands of Adventure Trading Company). This store seemed to have almost everything (but not EVERYTHING) they had in the smaller stores back in the WWOHP. There are also some limited choices at the store at the front of Universal Studios (aptly named Universal Studios Store). And lastly, there is a store at Universal CityWalk, (called Universal Studios Store also) where you can get limited WWOHP items.
Another note, you can purchase items at any store in the parks and have them sent to the front of the park for pick-up. You will receive a voucher for your bag when you pay for the items and you must have the voucher to pick up your bag later. In Islands of Adventure, the pick-up location is at a special desk located in the Islands of Adventure Trading Company. Our personal experience with this wasn't the best. The first time we did it, they didn't have our bags yet, and it had been 5 hours since the purchse. The second time was much smoother, but the wait was fairly long for the pick-up desk. But in the end, if you really need to buy your items in the WWOHP, it's a good idea to have them sent to the front. A lot of rides in Universal really aren't convenient to have bags with you and the lockers they provide are small and can have VERY long lines to deposit and pick up your things.
Sorry for the super long post - I hope it helps you! Enjoy your visit to WWOHP. It was really well done and such a treat for our family!