Amazon seller Fancy Dress Discount Store has the classic Elsa dress available for £19.95 delivered in sizes 5 - 8 years. (Unfortunately size 3- 4 is nearer £50, and very low stock). It's not the best price we've seen the dress, but it's a helluva lot cheaper than some other options we've seen!
Elsa's dress is described as a "pale blue dress with sheer sleeves and a large Elsa character print on the front". It's 100% Polyester and sponge clean only.
One of the good things in the reviews is actually the delivery, and that it's so speedy, so if you've been waiting to find these in stock somewhere, your wait at least shouldn't be too long before you can see the sheer delight on your little one's face. Seriously, the amount of mums I've heard talk about how hard this dress is to find - if I was into sewing I'd start making them!
As always with Amazon, prices can go up without notice, and we don't know what stock levels are like, so if you want one, don't take too long - just order. Your small will love it.