Assuming you mean a lens for a video camcorder, they sell $50 basic fisheye lens on eBay.com  But, I'm pretty sure you could find a higher quality lens off of refurbdepot.com though.  Just in case you're including still-shot photo lens, there's this spy lens that let's you shoot the camera in front (like usual) but, takes the picture from your direct left or right.  Using mirrors inside of the lens, you can capture unexpected moments from your friends, or your friend's girlfriend (whatever you want is your deal).