If you'd like a spare silicone case for your iPhone 3 or 3GS, 7Dayshop have a buy 1 get 1 free deal right now. You can get 2 cases for £0.79, with free delivery as well.
7Dayshop say that these silicone cases are designed for the 3G or 3GS, but that they also fit the original iPhone (with a little leeway). There's only one problem in my opinion - one of the silicone cases is pink! If you're a pink hater (like me), then you might not be interested in this deal at all. Or you could order it just to have the other case, which is clear in colour - much easier on the eyes.
40p per case is dead cheap. Taking a quick look on Amazon to see the kind of price ranges on similar cases, you're talking at least £2 each, if not more.