f you have Vitamin C tablets, you can crush them up and dissolve them in water... I know of some winemakers who do this. While lemon juice would definitely contain some ascorbic acid, it also contains alot of citric acid and will therefore increase your TA considerably. I suppose you could add the lemon juice BEFORE adding any acid, then test your TA and add enough acid to bring it up to the proper level. Ascorbic acid is added as an antioxidant and so you can omit it provided you add sulfite and try to handle the must as protectively as possible. (ie. ferment at a cooler temp., avoid splashing and aerating, and rack and top up promptly when SG dips down near 1.000)
You may even want to consider avoiding ascorbic acid in all your wines... there is some new research out of South Africa and Australia that suggests ascorbic acid might act as a pro-oxidant rather than an antioxidant under certain conditions. I might use it in some whites and fruit wines, but by no means should it be used in reds, or other wines with large concentrations of tannin and anthocyanins (pigments.)