I just ran a quick search using "Ariel Costume" on ebay. Lots of nice looking costumes reasonably priced. If purchased & paid for right away with paypal, you should be able to get it shipped and delivered in 2-3 days.
You'll want to be sure to let the seller know that you need it ASAP though to be certain they can ship priority mail and will get it right out.
DD6 has several ariel costumes that she loves to wear in the parks. Our "
DCL souvenier" it would figure Ariel would be her favorite. The bikini top w/fins is the one I think she has the most trouble wearing as she always seems to be tugging it back into place. Is her heart set on that particular one?
In person the WDW ariel wedding/bride costume with the multi-layered skirt in white, gold and aqua was a particular favorite and seems to sell very well in the parks. It was a must have the year before last and even though it doesn't have fins DD6 still considers it her best costime.
My personal favorite though, so far, is last Halloween's
Disney Store Deluxe version. It has a built-in seaweed cape, shimmery irridescent panels, and glittery fish-net centered down the skirt before the fins. I think it was originally 79.50 in the store but they were reduced a while back after Christmas so they are still found on ebay more reasonably.
When we were there in March we discovered the new Minnie Mouse Costume for the "Dreams" theme. It has a blue velvet bodice with Minnie on it and a full princess skirt. It matches the bigger one Minnie Mouse wears in Epcot (only Epcot, we were told even though the dress was sold all over wdw). I'm told it is hard to keep in stock in the small & medium sizes because it so popular. I know we had to wait while they sent someone to bring the last two size smalls up from inventory the day we bought it. That's one I'd call wdw merchandise to order ahead of time.
It's pricey, but the really special feature about this is that the new blue minnie dreams dress has built-in fiber optic light strands in the skirt. Absolutely captivating! It turned heads everywhere we went and even got her noticed & interviewed by the monsters during the new Monsters Inc Comedy Club Show as it twinkled in the audience - okay, i confess, we parental units were trying to hide as the camera and microphone zoomed in for her 60 seconds of fame but DD loved every moment of it.
DD6 got up every morning and gravitated right to it. On the third morning, I told her big sister to hide the dress while we went down to breakfast or she may never have taken it off!
And I'm sure DD could care less. 
