Most commercial diets formulated for specifically guinea pigs should have some Vitamin C added to them. I know both American Pet Diner and Oxbow do. I personally don't really think there is a good long term substitute for fresh sources though.
yes most diets should contain plenty of vit c as long as you use product within 90 days of manufactured date. i also use effervasant vit c in the water bottles. a lot of greens also contain vit c.
__________________ christina and the bluenose rats and other critters