Advice please - where best to buy replacement speakers?

[Greenwich Ross]

It would appear that one of my speakers (door mounted) has gone pop. It sounds like it might be torn as it buzzes badly whenever it's sent some bass. Listening, as I do, to a lot of dubstep this is annoying.

So, can anyone recommend somewhere that could sell me a replacement unit in the London/Kent/Surrey area or, alternatively, an internet site?

The car is a '03 Boxster with the Bose 'upgrade'.

Thanks

R.