35% USP food grade H2O2 is readily available.
It is also present in rain water at an effective level.
Food grade H2O2 is still produced the same way in factories. All food grade means is that they use less stabilizers in it. It does not mean that it is pure. Typically food grade is used for, among other things, electronics and propulsion. The opposite of food grade is cosmetic grade, that has extra stabilizers.
At 3-5ppm in rainwater, while this might be effective in neutralizing the .1ppm of sulfur dioxide often found in rainwater, and maybe contact killing some microorganisms and fungi, expecting it to do the same in vivo is a huge stretch. But saying so will not change the minds of those who are convinced of its anecdotal clinical efficacy.