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.