Try ebay - you might just get a mega bargain. If not then you could try New Look or H&M as they do have some nice items sometimes, and just dress it up a little with good accessories.
Bay Trading doesn't have the greatest selection of clothes but I bought a nice dress for a summer wedding this year - the shop is hit and miss but worth a shot.
Also, try Debenhams - especially Red Herring, they have a great selection and are not expensive considering they are a named brand.
Buttoneyes had a great idea - Red Herring at Debenhams. They have a really cute green flowery dress at the moment for �30. Its button up and with a tan belt looks really cool. Its not really really girly flowery, quite a vintage look. Black background with little green and white flowers on.
I'm wearing mine at the moment and have been getting lots of compliments on it. Suitable for work with flats and boots or for evening with heels or heeled boots