Expensive - http://www.hideawayknife.com/main.php
Not expensive - http://www.kabar.com/knives/detail/76 Also comes in Big and Little.
They are not so much for getting a person off of you (although they can do a decent job) as much as getting something off your strong-side hand/arm. Also, they wipe clean very easily after being brought into use when you actually want to cut that hunk of food in front of you instead of sawing away at it with whatever the restaurant gave you.
The HAK is less recognized as a knife, even after being deployed. And it allows you to hold on to it while still using both hands. Not that I'm prejudiced - much.