As to sources, some is made from rose hips, but most are synthetic, being brewed and refined from corn or sago palm. Some specialty brands are made from tapioca, and some others sources. If the refinement process is well done, all but the most sensitive will not notice the difference.
Rose Hips are something else. Extracts of rose hips are rich in bioflavinoids, and are sometimes added to some forms of C. If you are taking a lot of C, the addition of this can cause allergy problems in those susceptible to the formation of allergies due to Leaky Gut Syndrome. (See Great Smoky Diagnostic Labs.)
In general, those people with candida albecans and such should take their Rose Hips separate from their vitamin C, and on a Rotating Diet.
Most tablets have binders to hold the tablet together, one kind of glaze to make it easy to swallow, bulking agents to make the tablet big enough to handle. These too, can cause problems for those who have allergies or are allergy formers. That is why I take my C in powdered form, mixed with water. I buy it in bulk powder form from one of our long term sponsors, a small company called Wholesale Nutrition, using both the pure Ascorbic Acid form, and the salt form known as C-Salts, when I think I am running low on some of the salts. (C-Salts does not include sodium. If you think you are losing sodium salts, they also have a Sodium Ascorbate.)
Wholesale Nutrition has Very good prices. It is run by Oscar Falconi, and is written up in a number of health books. I think I started buying from him in 1981, and met him after I came out to California in 1986. I helped set up some of his computers, set him up on the web, taught him how to make his own web pages, and we are good friends now. A year or so ago, he moved the business from his home in Saratoga, California, where he had run it for over 25 years, to a small store front. He has good stuff at good prices. Dr. Cathcart buys from him too, re-selling some of his products along with many others in DocC's store. (Banner below.) These two are wonderful people who helped save my life, and have helped the lives of many other people. That's why I don't charge them for their web space.