There are some atkins ones but I'm not crazy about them. GNC is probably a good place to look. Have you considered making a low carb smoothie/shake at home and taking it to work or hard boiling some eggs in advance? They are nice and portable.
After 3 years of searching, I have officially given up on finding a low carb cereal or bar. Either eat a small amount of Special K or have two scoops of Jay Robb's protein powder with 16 oz. of water. Quick & easy & portable - I keep the powder in snack size baggies and pour it into a bottled water.