Berries, broccoli, green leafy vegetables, carrots and turnips all contain vitamin c, some have more than citrus fruits as well. One does have to remember that humans have been able to access all sorts of nutrients through more than just what we are told today have the particular nutrients in them. I thought for ages that citrus has more vit c in it, but it turns out kale has more.

Lots of common Western or Northern European foods have vitamin C, including chives, parsley, radish, kale, cabbage, mustard, broccoli, Brussels sprouts, all types of berries - the list is very long. An average medieval diet, while not nutrient dense by modern standards, would have had more than enough vitamin C.

Further, it was not very far to the Mediterranean, where all manner of tropical fruits and vegetables could grow - even in the south of France, Northern Spain and coastal Italy.