Depending on how high your expectations regarding quality and range are, there are cheap miniature wireless cams, not webcams, for under $50.00. Check Sparkfun, ebay, or just google miniature camera. Good luck and be sure to post some video. I am thinking of buying one as well.
thanks. i'm considering buying one for my robot(my first one). i suck at building robots, but i love to see the drive and to drive them.( i am reatarded i know)
For much less than the price of a webcam you can buy a decent nanny cam and a video in card for your pc.
I edited your subject line as "Where can u" doesn't describe what you are asking. This way others can be helped too...
Anyway to answer your question I would search ebay for wireless nanny cams or wireless web cams. Web cams are generally networked and will run over $100 easily. Wireless nanny cams don't quite have the range, but can run off a 9 volts and weigh a few ounces. The image isn't really great, but it is good enough to get an idea what the robot is seeing and most refresh at 30 FPS which is about the same as a video camera.