I was told the same thing

We too are looking for the Rapunzel dress in size small (5/6). I was able to buy the size 4 for my daughter but I had to return it becasue the arms were a little tight and too small on her.
Fortunately for us our trip isn't until September so I'm hoping to pick one up in August when they come back in stock for Halloween. I feel bad for those looking for them now though

IMO, the
Disney store version is absolutely gorgeous by the way...I liked it better than the pics of the "official" WDW version.