We buy 1kg containers of sodium ascorbate from Bronson, item 50B (but their prices have recently gone up, haven't compared to NOW in a while). I administer it directly to my kids, and when my son was little, I mixed a bit in water and used an oral syringe (the kind from the grocery store) to squirt it into his mouth, dosing at least 3-4x/day. Bronson ships pretty fast, I've been impressed with delivery times. I can also get much smaller containers of sodium ascorbate from Whole Foods, there's one brand that WF carries that makes it (not always there though).
You may want to dose yourself as well, to bowel tolerance. I'm pretty sure I haven't had it in years and years, so I expect the next time I encounter it, I will get a significant case, unless I do quite a bit of work to support my body through it. Hoping you do better than I expect to.
IIRC, the dosing for WC is 375mg/kg body weight, but I would use that as a starting point and consider going up until you hit bowel tolerance (diarrhea, I mean). For other illnesses (no WC here yet) my kids need more SA than normal dosing guidelines would suggest, so keep it in mind.