Magnesium ascorbate powder is an option. Actually my favorite, but I cant get hold of it anymore unless I order from US (and then it is held back by customs)
I have a variety of superfood like goji berries and fruits for more natural forms of vit C, (Tried amla for over a year but it didnt do the job and was to expensive in the long run)
In addition I take 3 grams of ascorbic acid, divided, daily; more if I need it.
To measure its effect; when I take enough I next to never have colds, flu or infections.
If I feel something is coming I double the dosages and the the flu or whatever leaves within hours.