Have you had a look on eBay? You might be able to pick up a second hand one there. I'd be inclined to stick to a proper Apple charger, just because of the scare stories you hear about exploding laptops and iPhones when the wrong chargers are used. Also make sure that it's the right voltage - don't buy a US plug if you are in the UK (and any knock off chargers are unlikely to be for our voltage as the market wouldn't be big enough to justify making it).
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