Watch this fantastic Suzanne Humphries Vit C video.  Watch the 1st one.  For the second video fast forward through their set up to 7:20 if you’d like some additional practical recommendations.
http://drsuzanne.net/dr-suzanne-humphries-oral-intravenous-vitamin-c/

Almost all vitamin C is made from GMO corn which is produced in China. China has a poor reputation for quality control. If a label says “non GMO” and “corn free” it is probably made from GMO corn unless the label states a specific source (such as tapioca). Manufacturers know that consumers often want to avoid GMO's, and since they are not required to list the source material on their label, they don’t. Once the GMO corn is processed into ascorbic acid, they can legally say their product is GMO free and non corn (because it is now called “ascorbic acid”). They say that there is no detectable GMO or corn antigen residue in the final product, though very sensitive people may do better with a non corn based product. Suzanne Humphries likes the Nutribiotic Sodium Ascorbate, which is inexpensive, though made in China from GMO corn. Nutribiotic says that they only contract with 2 companies in China who they trust. This is a step up from most manufacturers who buy on the open market from a large number of Chinese companies.

This is what she likes:
Nutribiotic C is sodium ascorbate (buffered with baking soda).
https://www.amazon.com/Nutribiotic-Sodium-Ascorbate-Powder-Ounce/dp/B0017V21ZS/ref=sr_1_7_s_it?s=hpc&ie=UTF8&qid=1546029021&sr=1-7&keywords=sodium+ascorbate+powder

The following 2 are ascorbic acid that need to be buffered.  Use health food store quality baking soda at a ratio of 2 parts C to 1 part baking soda (we actually prefer 1 part C to 3/8 part baking soda). Bob’s Red Mill is the best quality baking soda we know of and is commonly available. We like the Ecological Formulas ascorbic acid derived from tapioca (non corn, though made in China from approved manufacturers). You can use this affiliate link, then search for Ecological Formals ascorbic acid "powder”.  https://wellevate.me/john-holden

Daniel Cobb likes the Quali-C best, it is less expensive than Ecological Formulas and is manufactured in Scotland from conventional corn. This has better quality control (China Free) and is non GMO.
https://www.amazon.com/Doctors-Best-Vitamin-Quali-C-Scotland/dp/B00HNS1E0W

For liposomal Vit C, Suzzane Humphries likes the LivOn Labs Liposomal C, but it is made in China from GMO corn, has GMO soy lecithin and a chemical preservative.  Liposomal C can be more easily digested by many.  It has a higher cellular absorption rate in this form.  Therefore, it is attributed to be 6 times as potent as a dose of regular vitamin.
We like LipoNaturals Liposomal C liquid which uses Quali-C, sunflower lecithin and fermented prune. We usually have it available at the office, or you can purchase it online here at our affiliate link LipoNaturals.