I use Walden Farms dressings which are low in carbs- less than 1g per 2tbsp serving and vary in calories from 20 to 40 per serving. They offer Italian, Ranch, Thousand Island, French, etc...
I think they taste good, and they even make low-carb ketchup. If you can't find them in your grocery store, they're offered at many on-line low-carb sites. Good luck.
God Bless,
Amy-Lynne