have you tried somewhere like topshop, newlook and dorothy perkins? or maybe jane norman!!!! but it may be difficult to find a polka dot dress as they were in, in the summer and it is autumn/winter!!!!!!!!!!!!!!!!!!1
Ive got one from Primark which is almost identical (its more of a shirt dress) I always get comments that I look like Gwen in it, must be the blonde hair!
It was featured in the press over the summer as they sold nearly a million of them, so if you try ebay you should be able to find one.