I did a quick google search and it looks like Baskin Robbins makes a sugar free sherbert. But even better... I found a recipe that you can make at home:
Courtesy of H-E-B Product Evaluation Center
1 package unsweetened Kool-Aid Drink Mix, any flavor
1 quart H-E-B Fat-Free Milk
1 quart low-fat plain yogurt
24 packets Hill Country Fare No-Calorie Sweetener
Combine drink mix, milk, yogurt and no-calorie sweetener in an ice cream freezer container; stir to dissolve sugar. Insert the ice cream-maker paddle and lid, and insert into electri**** cream maker. Fill outer ice cream-maker container with ice and rock salt according to manufacturers instructions. Turn on electric freezer and let it churn for 30 minutes, or until machine stops (mixture is frozen). Serve and enjoy. Pour remainder in a plastic container with airtight lid and store in freezer.