I found "UnSweeted UnKetchup" at my grocery in the healthy foods section. It tastes great and has one carb per tablespoon, with no sugar added.

It's not as sweet as "tomato syrup", but the longer I am on the diet the sweeter it tastes .

I recall seeing at least one other option for unsweetened ketchup. If you don't have a good health section in your grocery, try a large health food mart.