I have a new package of instant yeast and there are really no ingredients listed (a commercial package) but it does have vit c listed in the nutritional label.

How about developing a sourdough starter? That is just using flour and water to culture the yeasts already present in the grain and air. There is quite an active sourdough forum on this site as well as a few others.

What do you want to make with the yeast?

I'll be making various types of bread.Vitamin C shouldn't be a problem.I've now found Rapunzel yeast which I will try .This is supposed to be additive free.