I don't know how it specifically interacts with COVID-19, but most modern people are vitamin D deficient, it's easy to supplement, and higher vitamin D levels than without supplementation seem to be correlated with better immune system function.

Not medical advice but my personal view

The Oura trials suggest that it can often detect illness 3 days before showing clinical symptoms. I personally would take an unexplained fall in HRV/high bodytemperature/higher resting HR as a sign to take more measures.

When it comes to zinc lozenges for the flu you need to take them very early to be effective. If you wait for a test result you are likely past the point where the zinc lozenges do much. 

Having half of a day before you are even officially tested a zinc lozenge in your mouth isn't fun but it might be what's required to see the protective effects.

Neti pots provide an additional treatment that I would use because of the literature of how it helps with influenza. Of course you need to take care to properly clean a neti pot, use sterile water and the right salt content. 

Drinking a lot of water (maybe ~4 liter), eating lightly and sheduling a lot of sleep seem also decent.

Disclaimer : I have not researched any guidance for self care, just sharing one reasonning here.

I have no knowledge about this whatsoever, but from a population standpoint I'd be surprised if there was anything that you could buy that will probably help you while not posing comparatively more risks to others by the simple act of going shopping while positive (thus spreading).

What I mean is that I think everyone in the area is better of if they isolate as soon as they know they're positive instead of first going shopping for something that might help.

Of course your question still holds if you're wondering this before being tested positive, which you are doing now.

Again : not qualified, but I'd say that rest and proper sleeping positions are probably good whereas taking moderate to large doses of whatever supplements (zinc, vitamin C etc) could definitely have slighty worse effects.

My reasonning is that for supplements : the expected positive gain is small otherwise it would already be known. The possible negative impact of supplements is also unknown, but I think that most won't take the same supplements that you do so it could have worsenning effects without being known and documented.

I wrote this because the thought seemed worth sharing and I'd appreciate feedbacks and correction on this, but I sincerely hope it won't be read as intended to stress you out or something, as you might be in a bit of a stressful situation.

From bipedal to (probably) bipedal : wish you good luck.