eBay is cheapest, although you might find that they are not particularly good quality and tend to fade. If your MoT tester, or Traffic Plod is being REALLY strict they might even say that they're invalid if they don't have the correct font/spacing and the plate makers details on it.
A lot of motor trade shops (including Halfords) do them, although you do need to take ID for the car (log book) and yourself (driving licence) before they will do them for you.
Gov.UK