Tesco, ASDA, Sainburys, Lloyds and Boots do not report carbs on vitamin packs.
From Google:
Nutritional Information, Diet Info and Calories in
Dietary Supplement, Vitamin C 500 Mg Chewable Tablets
Per tablet: 2g carbohydrates
Nutriton facts Vitamin C 500 mg Per tablet: 0.5g carbohydrates
These carb amounts are small, except perhaps for some extreme keto-diets.
The main ingredient of a vitamin C tablet is ascorbic acid, so it probably does not matter where you buy.
A chewable Wilko 500mg Vitamin C tablet contains:
Sucrose, Sodium Ascorbate, Ascorbic Acid, Maltodextrin, Stearic Acid, Magnesium Stearate, Hydroxypropylmethylcellulose, Silicon Dioxide, Maize Starch, Flavouring, Modified Maize Starch.
(sugars and sweeteners in list)
Wilko Slow Release Vitamin C 500mg contains:
Ascorbic Acid, Dicalcium Phosphate, Hydroxypropylmethylcellulose, Cellulose, Silicon Dioxide, Magnesium Stearate, Stearic Acid, Glycerol, Talc, Glazing Agent
(do not know if any of the above ingredients are/contain sugars or sweeteners)
Links:
https://www.fitbit.com/foods/Dietary+Supplement+Vitamin+C+500+Mg+Chewable+Tablets/14733971
https://www.myfooddiary.com/foods/7265835/nature-made-vitamin-c-500-mg