Where do I get a new lens cap?

For a Nikkor 18-55mm?

You can buy them at any proper camera store.

You can get much cheaper 3rd party ones on eBay, including ones with the Nikon logo. They are listed in filter thread sizes.

Either online or a camera store.
Make sure its the right size.

It requires a 52mm cap. Here is the original Nikon cap, but you can find generic brands lots cheaper.

http://www.bhphotovideo.com/...s_Cap.html