I agree with Lilylogan on little adventures. I bought 2 Moana dresses from little adventures for 2 different families and both girls loved them. It was one piece and comfortable to wear all the time (which they did). My daughter had a 2 piece Moana dress from Disney and it just never looked right and she didn’t like it. 1 piece worked much better for the little girls that I bought for.
I also vote for Little Adventures. Or, just go to Ebay, search "Costumes" with the words "Moana", and then sort low to high price. I bet there are dozens on offer!