Nikon 18-55mm front lens cap hood

I got mine from eBay and it costs around £2. It's a 52mm lens cap hood (LC-52) in diameter and you can get a cheaper replacement one for less than a pound (it won't be Nikon own brand though, something different).

Since the Nikon lens cap hood is not expensive, you might want to go for this one rather than an unbranded one. I like the idea of having the little Nikon logo displayed on the lens cap as it gives more credibility to my lens but that's just me :)

Try contacting Nikon directly and asking them how much a replacement costs. It may be quite expensive so you might want to try the many websites that specializes in spares and replacement parts. If that also fails then try eBay or Amazon as you will that that they are also very competitive