The Superwalmart on Vineland has a huge selection (think this is the Kissimee one). It's been posted that there is a section at the SuperTarget as well.
I usually make a stop there and since I tend to do small things like fridge magnets and keychains it's a great place to pick up something inexpensive. I mean really I don't want to spend more than 2 or 3 bucks on a magnet for my desk at work. This last time I bought a picture frame after failing to find one I liked anywhere at disney on two prior trips. I think my mom bought a shirt there a few years ago and still wears it so it held up well. I don't really care about the 'official' stamp on my souvineers and honestly for as much as you pay IN WDW some of the quality is lacking to say the least. So take a look and decide if price/quality is workable. In truth I say for anything that your kids will grow out of or is something that is not a show piece item then cheaper is better. For those few items that are show pieced or things you want to last you might be better off at an official store.
It's not the only place I buy stuff but it's nice for things that I really do want to keep budget.