I very much doubt you can buy spare parts for things like this when you can replace it for less than £9.
If you want to make it a project I expect you could make your own spring, but personally I wouldn't bother.
http://www.argos.co.uk/static/Product/partNumber/0153775/c_1/1%7Ccat_14418476%7CSmall+kitchen+appliances%7C14418587/Trail/searchtext%3ETIMER.htm
Adam, September 2009