Maxi dresses are very in at the mo so you should be able to get hold of one in any mainstream fashion chain; Miss Selfridge, TopShop, New Look, River Island, Primark etc etc. Some are also online if you find it hard to get to the shops. ALternatively, get a vintage one on eBay, x
Dorothy Perkins have some nice maxi dresses due to arrive in store and on-line imminently. Their web-site is beginning to advertise them as 'forthcoming' now. I think that they are going to be between �20 and �25. Hope this helps. BB x