I would have a look at bridesmaid dresses - teh tend to be cheaper, plainer and you can get them in any colour you want usually - I didn't want anything too formal for my wedding, so I had a bridesmaids dress made in ivory and it cost a quarter of the price, was really simple and was very much admired - the shop even said that they had never thought of doing that particular dress in ivory, but thought that it had worked really well.
Other than that, your best bet would be somewhere like Debenhams or John lewis or Frasers who would do evening gowns, but getting it in a suitable colour meay be more of a problem.