If it's just the battery, you can get one from Halfords or a Renault Dealer. However, when I had the same problem it wasn't the battery. I had to get a new key from a Renault Dealer, which was about £80. The type of key varies with the model and year, so yours may cost more or less, and may be available over the counter or have to be ordered from the factory.
Dan, May 2008