They had a Universal Store at the Orlando airport. Or we purchased a wand for my son's birthday from their on-line site. Though someone did tell us to check out Amazon too. They seem to have everything and usually a lot cheaper!
I got some very unique t-shirts at the Warner Brothers site to bring with us for our trips to US in November... My DD13 LOVES the Weasley twins and they have a tshirt that is an ad for their joke shop - she was thrilled about it! Get on their mail list, they're always having specials, particularly on HP stuff.
This is only helpful if you're looking online of course