It was not available 2 weeks ago when we were there. There was very very limited Frozen merchandise. They might be getting more dresses in tomorrow for the DVD/Blue Ray release?
Depending on size, it's been sold out on the website since 8:20 am. Every time I put one in my cart it vanishes when I go to check out. There was no limit on the dresses (vs. the dolls.)
The fact there is anything in the parks though gives me hope that at some point we will actually successfully get one.