Unfortunately, I have never seen a full on Stitch costume being sold at the parks. However, when I was 9, which would be 11 years ago, WDW did sell some really cute and handy Stitch backpacks. I still have mine sitting on my bed.
I don't know if they still sell them, though. I last went to WDW when I was 18 and I don't remember having seen them twice.
I'm pretty obsessed with Lilo & Stitch stuff, myself. I still get on Youtube and watch the animated series that was on TV when I was a kid and I still watch the VHS I have of the first movie a lot. Sometimes, I even cuddle up with my Stitch and put on my Experiment 624: Angel kigurumi (Japanese pajamas).
If you can find one in her size online, she would love the Stitch kigurumi. Or, if you know anyone that goes to anime cons, they might be able to find you one at one of them. We have a local anime convention that I go to nearly every year and one of the dealers almost always has Stitch and Angel kigurumis. I bought my Angel one from him.
Angel is the pink one, if you can google the image of both pajamas. My boyfriend might be getting the Stitch one so we can wear them to Halloween at
Disneyland this year. And it fits because Angel is Stitch's love interest, although she is mainly a TV show and sequels character. Angel is not in the first or second films.
I would include links to images of the backpack and the pajamas, but since my post count is lower than 10 right now, I'm not allowed to post the links.