You could try
Ebuyer, which offers OEM versions of many brands of security software, including Norton. If you want to stick with Norton, you could go for Norton Internet Security, which incorporates AV / Firewall / Anti-Spyware and Anti-Spam, which would cost you round about £25 plus a bit of postage, but if you fancy a change, I would recommend you try F-Secure Internet Security (OEM), which is about £10 cheaper than Norton.
One thing to say about F-Secure is, you would be required to uninstall all other security software from your PC, and you would definitely have to ensure that all traces of Norton are removed, by using the
Norton Removal Tool (there is an updated version for Norton 2007), but I have found F-Secure to be very satisfactory, at a lot less than the cost of Norton.