I am not sure what kind of shops you have in the UK, but here in the US most any music shop or place like a Radio Shack will have them. Almost anywhere you can buy decent headphones will have them these days too.
Any larger supermarkets with a home electronics section have these in the cables & connectors shelf.
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"..one day when someone comes up to you asking for advice you realize that it's never been the equipment at all." - Ken Rockwell, photographer