Boohoo.com have some lovely maxi dresses - I've bought 2 recently and I don't wear dresses usually - they really cheap too mine were £15 each
I also got a pretty maxi dress from eBay - it took 2 week to come because it was from hong kong but they had some beautiful designs for less than £20 (including p&p)
Try ebay and type in for example, maxi dress size 16 (for example)
I buy lots of clothes off ebay for great prices. I love next clothes and cant always afford their prices so i ebay: Next size 16 dress (for example) and ive bought loads!