Where do you guys buy your vitamins?

I'm looking for advice on how to source a high-quality form of Vitamin C in bulk, specifically a 1kg supply. For instance, I’ve noticed that if someone wants to take significant doses of Vitamin D3 (anything over 5,000 IU...), it’s almost impossible to find without paying a massive premium. There is a Plivit D3 where a single drop provides 200 IU, but I have a feeling that dosing at such high concentrations could actually become toxic due to all the added fillers and additives. It's honestly pretty unpleasant to drink, though you can technically get it via prescription. On the other hand, there's an Oleovit Vitamin D3 supplement that contains absolutely no additives—just pure Vitamin D3 in castor oil. A 5,000 IU dose lasts about a month and costs only $10(I order it from Amazon, which ships from Germany...)

You definitely won't find any answers here... those gym rats over at Gold's Gym are probably obsessed with all that stuff, so they'd likely be your best bet if you're looking to dive deep... whew...

First off, what’s the deal with wanting to chug all those vitamins? Who actually told you that you need them or that it's even safe to go that hard?